Photocyclisierungen von 1, 1'-Polymethylen-di-2-pyridonen. 46. Mitteilung über Photoreaktionen

Photocyclization of 1, 1′-Polymethylene-di-2-pyridones . Benzophenone sensitized irradiation of the four dipyridones 1-4 gave the internal photocyclization products 6 (64%, Scheme 4), 7 (60%, Scheme 5), 8 (Scheme 6), and 11 (26%, Scheme 7), respectively. The decamethylene compound 5 yielded only polymeric material. The primary [2+2] photoproduct 8 from dipyridone 3 (Scheme 6) is relatively unstable. Further irradiation or heating to 65° induced a Cope rearrangement to give compound 9 which, on heating to 137°, was converted into the isomeric compound 10 . This product, as well as the other photoproducts mentioned, are rearranged back to their respective starting materials upon direct irradiation with 254 nm light or by heating to higher temperatures. The various possibilities for cycloadditions of pyridones are discussed as well as the possible factors which are responsible for the highly regioselective photoreactions of the dipyridones 1–4 .     

Photochemische Reaktionen. 104. Mitteilung [1]. Zur Photospaltung der C,C-Oxiranbindung konjugierter γ, δ-Epoxy-enone: Photolyse von 4-Methyliden-5,6-epoxy-5,6-dihydro-β-jonon

The photoinduced cleavage of the C,C-oxirane bond of γ, δ-epoxy-enones: UV.-irradiation of 4-methylidene-5,6-epoxy-5,6-dihydro-β-ionone On ¹n, π*-excitation (λ ≥ 347 nm, pentane) 5 gives the isomeric bicyclic ether 10 in 75% yield (s. Scheme 2). In methanol the photoconversion of 5 to 10 is strongly reduced (12%) in favour of the formation of the methanol adduct 11 (43%). On photolysis in aqueous acetonitrile 5 is converted to the bicyclic ether 10 (9%), the dihydrofurane 12 (18%) as well as to the triketones 13A and 13B (7%), and 14 (23%). On ¹π, π*-excitation (λ = 254 nm) in pentane no 10 is formed, but 5 isomerizes to the tricyclic cyclopropyl compound 16 (59%), the allenic product 17 (10%), and the cyclopropene compound 18 (12%; s. Scheme 3). Photolysis in methanol furnishes 11 (63%), and 18 (4%), but no tricyclic cyclopropyl compound 16 . In a secondary photoreaction (λ = 254 nm) the dihydrofurane 12 is isomerized to the bicyclic cyclopropyl compound 20 . Evidence is given that the products 11 and 13 are formed by solvent addition to an intermediate ketonium ylide b (s. Scheme 12). The presence of b is further proven by the formation of 12 , a product of an electrocyclization of b . On photofragmentation of b carbenoids d and e are presumably formed (s. Scheme 14). 1,2-Hydrogen shift in d yields the allene derivative 17 , and cyclization of d gives the cyclopropene compound 18 . On the other hand, e cyclizes to the non isolated cyclopropene compound 69 which is transformed to 16 by an intramolecular [4 + 2]-cycloaddition. The present investigation shows that the photochemistry of 5 is determined by photoinduced C,C-bond cleavage of the oxirane ring. This is in sharp contrast to the photochemistry of conjugated γ, δ-epoxy-enones without the additional double bond in ε, ζ-position, where selective photocleavage of the C(λ), O-bond is observed.     

Indirect Photo-Induced Phosphorylation Via a C-Ester Caged Troika Acid

Abstract

Use of a photoremovable “caging” group allows the generation of reactive molecules under mild conditions. Photo-induced phosphorylations typically have involved attachment of the photosensitive group at phosphorus.[1] We now have investigated indirect photolytic activation of an unmodified phosphonic acid group using broad band UV (Hg lamp), 308 nm XeCl excimer laser or 355 nm YAC laser irradiation of the o-nitrobenzyl C-ester of “troika acid” [(E)-1²]. In alcohols or neutral buffer, irradiation of (E)-2 gave phosphorylation of the solvent plus phosphorocyanidate, the expected 2-isomer product.²All thrce UV sources gave -1:2 E:Z product distribution in MeOH. In the (E)-1 methyl C-ester, the oxime functionality absorbed strongly near 205 nm (E_max 5200), weakly at 308 nm and negligibly above 355 nm, and no photoisomerization was seen using the 355 nm source. Thus, oxime isomerization in (E)-2 at least using 355 nm irradiation. requires the o-nitrobenzyl group, and possibly involves an energy- or charge-wansfer effect. Phosphorylation of EtOH/t-BuOH mixtures by photolysis of (E)-2 showed little alkyl selectivity. consistent with photoinduced formation of an intermediate. plausibly (E)-1, which undergoes spontaneous dissociative fragmentation via a monomeric metaphosphate-like species.     

Glyconothio-O-lactones. Part I. Preparation and reactions with nucleophiles

Furanoid and pyranoid glyconothio-O-lactones were prepared by photolysis of S-phenacyl thioglycosides or by thermolysis of S-glycosyl thiosulfinates, which gave better results than the thionation of glyconolactones with Lawesson's reagent. Thermolysis of the thiosulfinates obtained from the dimannofuranosyl disulfide 7 or the manofuranosyl methly disulfide 8 (Scheme 2) gave low yields of the thio-O-lactone 2 . However, photolysis of the S-phenacyl thioglycoside 6 obtained by in situ alkylation of the thiolato anion derived from 5 led in 78–89% to 2 . Similarly, the dithiocarbonate 10 was transformed, via 11a , into the ribo-thio-O-lactone 12 (79%). Thermolysis of the peracetylated thiosulfinates 14 (Scheme 3) led to the intermediate thio-O-lactone 15 , which underwent facile β-elimination of AcOH (→ 16 , 75%) during chromatography. The perbenzylated S-glucopyranosyl dithiocarbonate 18 (Scheme 4) was transformed either into the S-phenacyl thioglucoside 19 or into a mixture of the anomeric methyl disulfides 21a/b . Whereas the photolysis of 19 led in moderate yield to 2-deoxy-thio-O-lactone 20 , oxidation of 21b and thermolysis of resulting thiosulfinates gave the thio-O-lactone 4 (79%), which was transformed into 20 (36%) upon photolysis. The pyranoid manno-thio-O-lactone 26 was prepared in the same way and in good yields from 22 via the dithiocarbonate 24b and the disulfide 25 . The ring conformations of the δ-thio-O-lactones, flattened ⁴C₁ for 15 and 4 and B_2,5 for 26 , are similar to the ones of the O-analogous oxo-glyconolactones. The reaction of 2 (Scheme 5) with MeLi and then with MeI gave the thioglycoside 27 (29%) and the dimeric thio-O-lactone 29 (47%). The analogous treatment of 2 with lithium dimethylcuprate (LiCuMe₂) and MeI led to a 4:1 mixture (47%) of 31 and 27 . The structure of 2 was proven by an X-ray analysis, and the configuration at C(6) and C(5) of 29 was deduced from NOE experiments. Substitution of MeI by CD₃I led to the CD₃S analogues of 27 , 29 , and 31 , i.e. 28 , 30 , and 32 , respectively, evidencing carbophilic addition and ‘exo’-attack on 2 by MeLi and the enethiolato anion derived from 2 . The preferred ‘endo’-attack of LiCuMe₂ is rationalized by postulating a single-electron transfer and a diastereoselective pyramidalization of the intermediate radical anion.     

Photochemical Behavior and Photolysis of Protonated Forms of Levofloxacin

The effect of intermolecular proton transfer on the spectral properties of levofloxacin in the ground and excited electronic states was studied. The preferred direction of possible protolytic reactions induced by UV irradiation in this compound was studied. It was found that the proton transfer processes have a considerable effect on the capability of the compound to emit light and occur on the nanosecond timescale. The photochemical reactions of the tree forms of levofloxacin (pH: 4.0, 7.0, 10.0) were studied by laser flash photolysis and product studies. Irradiation at pH 4 yielded a pulse and transient (λ_max = 395, 515, 575 nm) assigned to the protonated triplet. Irradiation at pH 7 yielded a transient species (λ_max = 525, 610 nm) assigned to the neutral form. Protonation of the anionic singlet excited state was also observed (λ_max = 440, 570, 680 nm).     

Photo‐Cross‐Linking of Polymethacrylates with Stilbene Chromophores in the Side Chains

Methacrylates (=2‐methylpropenoates) 5 with (E)‐stilbene (=(E)‐1,2‐diphenylethene) building blocks on tethers of variable length were prepared (Scheme 2) and polymerized (i.e., 5 → 6 ; Scheme 3) in the presence of AIBN (=2,2′‐azobis(2‐methylpropanenitrile). 4‐[(E)‐2‐Phenylethenyl]phenyl acetate ( 7 ) as model compound established the cyclodimerization as a single irreversible photoreaction. i.e., ( 7 → 8 – 11 ; Scheme 4) in the absence of oxygen. The solution photolysis of the polymers 6 provided a similar result, whereby [2π+2π] cycloadditions of stilbene units of neighboring tethers predominated. On the contrary, the desired photo‐cross‐linking of chaines occurred in the irradiation of polymer films.     

Asymmetric Synthesis of 3-Hydroxyprolines by Photocyclization of C(1′)-Substituted N-(2-Benzoylethyl)glycine Esters

The chiral N-(2-benzoylethyl)-N-tosylglycine esters 5a–h and the α-amino-γ-keto ester 6 were prepared from γ-(tosylamino) alcohols 7a–h . Irradiation of compounds 5a–c, e gave cis-3-hydroxyproline esters 20–23 (Scheme 6), partly with complete asymmetric induction by the C(1′)-substituent, whereas 6 gave enantiomerically pure 4-hydroxy-4-phenyl-L -proline esters 24 in good yield but low de (Scheme 6). The de of the photocyclization depended on the nature and/or size of the C(1′)-substituents. Irradiation of ketones 5d and 5f , bearing H-atoms at C(γ) with respect to the keto function, gave cyclobutanols (Scheme 9) in low yields besides the preferred Norrish-type-II cleavage product. Cyclopentanol 25 was a by-product of the photocyclization of 5c as a result of H? C(δ) abstraction from the t-Bu group. The structure of products 20, 22 , and 24a, b was established by NMR or X-ray analyses.     

Acid-Catalysed Dehydration of Tricyclic Unsaturated Alcohols

The tricyclic alcohols 3–7 , derived from the corresponding ketones 1 and 2 (Scheme 1), by action of acids underwent dehydration with skeletal rearrangements. Dehydration of 3 and 4 with POCl₃/pyridine (procedure A) afforded the polycyclic hydrocarbons 9, 10 , and 12, 13 , respectively. With TsOH (procedure B), on the other hand, 3 and 4 gave homo-triquinacenes 10 and 14 respectively, as well as the polycyclic ethers 11 and 15 (Scheme 2). Hydrocarbon 9 (or 12 ) was converted into 10 FSO₃H to the tertiary alcohol 16 (Scheme 4). Plausible mechanisms for these transformations are summarized in Scheme 8. Dehydration of the secondary alcohols 5 and 7 was effected by procedure A. While treatment of alcohol 5 with POCl₃/pyridine yielded two isomeric hydrocarbons 17 and 18 , similar dehydration of its epimeric alcohol 7 afforded hydrocarbon 21 as the sole product. The tertiary alcohol 6 was dehydrated by both procedures to yield two isomeric hydrocarbons 19 and 20 (Scheme 5). Hydrocarbon 20 was converted into 19 by procedure B (mechanisms, Scheme 10). Reaction of ketone 2 with CF₃COOH gave the addition product 22 converted into vinylsulfonyl fluorides 24 and 25 by treatment with FSO₃H (Scheme 6). Homo-triquinacenes 10 and 14 reacted smoothly with 4-phenyl-1,2,4-triazoline-3,5-dione to give the ‘ene’-reaction products 26 and 27 , respectively.     

Reactions of Tricyclic Vinylcyclopropanes with Metal Carbonyls

Irradiation of the tricyclic vinylcyclopropane 3 and Fe(CO)₅ resulted in the formation of the s?,π-bonded iron complex 7 and the π,π-bonded iron complex 8 (Scheme 2). Complex 8 was easily degraded with silica gel to give hydrocarbon 9 , which reproduced 8 by photolysis in the presence of Fe(CO)₅. Photolysis of 7 afforded a mixture of 3 (23%), 9 (27,5%), and three other hydrocarbons. Oxidative degradation of 7 with ceric ammonium nitrate in methanol gave the dimethoxy-hydrocarbon 10 . - The tricyclic hydrocarbon 3 isomerized thermally to the bicyclic hydrocarbon 11 (with CH₃? C(9) in an exo position) via a homosigmatropic [1,5]-H-shift. On the other hand, 3 was converted into the other isomer 14 (with CH₃? C(9) in an endo position) by action of Mo(CO)₆ or TsOH. Both isomers 11 and 14 reacted with 4-phenyl-1,2,4-triazoline-3,5-dione to give the isomeric Diels-Alder adducts 12 and 15 , respectively, which were photochemically converted into the cage compounds 13 and 16 , respectively (Scheme 3). - Photochemical reaction of the tricyclic vinylcyclopropane 6 with Fe(CO)₅ gave the σ,π-bonded iron complexes 17 and 18 . Heating of 17 at 80° resulted in a loss of one mol of carbon monoxide to give 18 in quantitative yield. Oxidative degradation of 17 with ceric ammonium nitrate in ethanol afforded the polycyclic lactones 19 and 20 by a novel type of reaction (Scheme 4). - The tricyclic ketone 21 was thermally converted into the α,β-unsaturated ketone 22 via a homosigmatropic [1,5]-H-shift. The configuration at C(7) of 22 was confirmed to be same as that of 11 (CH₃? C(9) in an exo position) by chemical conversions: 22 was reduced with NaBH₄ to alcohol 23 which, in turn, was dehydrated with POCl₃/pyridine to 11 (Scheme 5). Reaction of ketone 21 with Mo(CO)₆ gave the α,β-unsaturated ketone 25 and a cage compound X , whose structure was not fully elucidated. - Reaction of the polycyclic epoxide 26 with Fe₂(CO)₉ or Mo(CO)₆ yielded the allyl alcohol 27 in a novel type of reaction. The epoxides 29 and 32 were similarly converted into the corresponding allyl alcohols 30 and 33 , respectively (Scheme 6).     

Desoxy-nitrozucker. 2. Mitteilung Herstellung geschützter 1-Desoxy-1-nitroaldosen

Synthesis of Protected 1-Deoxy-1-nitroaldoses The direct oxidation of the oxime 1 with t-butyl hydroperoxide and vanadyl acetylacetonate yielding the nitro derivative 2 (54%, Scheme 1) could not be applied to other oximes. Diastereoselective bromination of the aldonolactone oxims 7 and 10–12 according to known procedures gave the corresponding bromonitroso compounds which were oxidized to the bromonitro compounds 9, 14, 18 and 22 , respectively. Oxidation of the bromonitroso compound in the D-mannopyranose series proved difficult, but the corresponding chloronitro derivative 23 was easily obtained according to Corey & Estreicher (Scheme 2 and 3). The structure of the bromonitro compound 9 was determined by an X-ray analysis, and the configurations of the bromonitro compounds 14, 18 and 22 were deduced from their molecular rotations. Reduction of the bromonitro compounds gave the protected 1-deoxy-l-nitroaldoses 2 , 15/16 , 19/20 , and 24/25 , respectively, in good overall yields. The ribose derivatives 15 and 16 were detritylated to give the nitro compound 4 , and the mannose derivative 2 was partially deprotected to give the monoisopropylidene compound 26 . The nitro group shows a normal anomeric effect which is reflected in the IR . spectra of the pyranose derivatives 19 and 20 , and 24 and 25 .     

Lacton-Bildung durch ringerweiternde Fragmentierung von Epoxycyclodecanon-Derivaten

Lactones from Epoxycyclodecanone Derivatives by Ring Enlargement Involving Fragmentation Reactions A stereospecific ring-enlargement reaction of alkyl esters of 2,3-epoxy-1-(3-hydroxypropyl)-10-oxocyclo-decanecarboxylic-acid derivatives is described, involving Grob fragmentation of in situ formed hemiacetals. The assignment of the relative configuration of the starting materials was accomplished on the basis of ¹H-NMR data. The rearrangement of the epoxides 9 and 10 (with cis-orientation of the ester group and the epoxide ring, Scheme 1) gives the lactone 15 as the single and as the major product, respectively, with (Z)-configuration of the newly formed C?C bond. A concerted reaction mechanism is assumed. The formation of a small amount of 12 from 10 is probably due to a competitive two-step carbanion pathway. The reaction of the diastereoisomers 7 and 8 leads to the lactones 11 and 12 , respectively, as the only ring-enlargement products (Scheme 1), with (E)-configuration of the newly formed C?C bond. On the basis of our results, we cannot distinguish in this case between a concerted and a two-step carbanion mechanism. This type of reaction takes place only in the presence of an ester group; no ring enlargement was detected in case of compound 20 (Scheme 3), which is the de(alkoxycarbonyl) derivative of 9 . The eliminative opening of the epoxide ring in the epoxylactone 17 affords 11 as the single product (Scheme 2). A carbanion mechanism was assumed for this reaction.     

Studies on Debenzylation and Photolysis of 1-Benzyl- and 1-(β-Phenethyl)-1,2,3,4-tetrahydroisoquinolines

Debenzylation of 1-(3-benzyloxybenzyl)-1,2,3,4-tetrahydroisoquinolines 1 , 6 , 7 with hydrochloric acid and ethanol gave the corresponding phenolic isoquinolines 2 , 8 , 9 and tetrahydroprotoberberines 4 , 12 , 13 . Compounds 2 , 8 , 9 on photolysis also gave, besides the expected noraporphines 3 , 10 , 11 , the tetrahydroprotoberberines 4 , 12 , 13 [1–4] (Schemes 1 and 2). 6-Benzyloxy-1-(5-benzyloxy-2-bromo-benzyl)-1,2,3,4-tetrahydroisoquinoline (27a) containing no methoxy or methylenedioxy groups either in ring A or C does not give protoberberine during debenzylation; but 28 , the debenzylation product of 27a , on photolysis gives both the noraporphine 29 and the tetrahydroprotoberberine 30 (Scheme 6), proving that during debenzylation of 1-(3-benzyloxybenzyl)-1,2,3,4-tetrahydroisoquinolines containing additional methoxy or methylenedioxy groups, the necessary formaldehyde comes from the latter groups. During photolysis both the methoxy groups (methylenedioxy groups) and the C(3) atom of the tetrahydroisoquinoline moiety provide the formaldehyde. Veratrole under debenzylation and photolytic conditions and tetrahydroisoquinoline under the latter condition also give rise to formaldehyde (Schemes 8 and 10). The novel bromohomoprotoberberine 43 along with 42 was formed during debenzylation of the 1-phenethyl-1,2,3,4-tetrahydroisoquinoline 41 . Photolysis of 42 yielded the novel nor-homoaporphine 44 , in addition to 43 ; the latter was debrominated to give the homoberbine 45 .     

Synthese von 2H-Isoindol-4,7 -dionen durch photochemische Cycloaddition von 2,3-Diphenyl-2H-azirin an 1,4-Chinone. 38. Mitteilung über Photoreaktionen

2, 3-Diphenyl-2H-azirine ( 1 ) reacts on irradiation with light of wavelength 290–350 nm with 1,4-benzoquinones 3–6 or with 1,4-naphthoquinones 7–9 forming the yellow to red coloured 1,3-diphenyl-2H-isoindole-4, 7-diones 10–13 (33–43% yield) resp. 1, 3-diphenyl-2H-benzo[f]isoindole-4,9-diones 14–16 (33–36% yield) (Scheme 1). The structures of these hitherto unknown products follow from the analytical and spectral data. The probable formation of the isoindole-diones is depicted in Scheme 2. The intermediate benzonitrile-benzylide ( 2 ), which most certainly arises, adds onto the unsubstituted C, C-double bond of the quinones and not onto the C,O-double bonds. On exclusion of atmospheric oxygen there results from 1 and 2-methyl-1, 4-benzoquinone ( 4 ) a product (probably b ) which hardly absorbs in the region 350–450 nm. The latter, with the agency of atmospheric oxygen (but not 4 ), is converted into 5-methyl-1, 3-diphenyl-2H-isoindole-4, 7-dione ( 11 ). The relative slowness of this oxidation (see Fig. 2) enables an almost complete photochemical transformation of the azirine 1 , which only weakly absorbs above 290 nm. Otherwise 11 , which strongly absorbs above 290 nm, would hinder the photolysis of 1 .     

Photolysis of Carbonyl Diisocyanate: Generation of Isocyanatocarbonyl Nitrene and Diazomethanone

The stepwise decomposition of carbonyl diisocyanate, OC(NCO)₂, has been studied by using IR spectroscopy in solid argon matrices at 16 K. Upon irradiation with an ArF laser (λ=193 nm), carbonyl diisocyanate split off CO and furnished a new carbonyl nitrene, OCNC(O)N, in its triplet ground state. Two conformers of the nitrene, syn and anti, that were derived from the two conformers of OC(NCO)₂ (62 % syn–syn and 38 % syn–anti) were identified and characterized by combining IR spectroscopy and quantum chemical calculations. Subsequent irradiation with visible light (λ>395 nm) caused the Curtius rearrangement of the nitrene into OCNNCO. In addition to the expected decomposition products, N₂ and CO, further photolysis of OCNNCO with the ArF laser yielded NOCN, through a diazomethanone (NNCO) intermediate. To further validate our proposed reaction mechanism, ArF‐laser photolysis of the closely related NNNNCO and cyclo‐N₂CO in solid argon matrices were also studied. The observations of NOCN and in situ CO‐trapped product OCNNCO provided indirect evidence to support the initial generation of NNCO as a common intermediate during the laser photolysis of OCNNCO, NNNNCO, and cyclo‐N₂CO.     

Synthesis and Screening of New Chiral Ligands for the Copper‐Catalysed Enantioselective Allylic Substitution

The synthesis of the new chiral ligands 6ae, 8ae, 9ae , and 11ae starting from the chiral β‐[(Boc)amino]sulfonamide 3ae is reported. The β‐amino group of 3ae was deprotected and condensed with 3,5‐dichlorosalicylaldehyde ( 4 ) to yield the known Schiff base 5ae , which was then reduced to the amino compound 6ae (Scheme 3). Alternatively, condensation of the free amino compound with 2‐(diphenylphosphanyl)benzaldehyde ( 7 ) afforded the imino ligand 8ae which upon reduction yielded the amino ligand 9ae (Scheme 4). The free amino compound derived from 3ae was also coupled with 2‐(diphenylphosphanyl)benzoic acid ( 10 ) to give ligand 11ae (Scheme 5). These ligands were tested in the copper‐catalysed allylic substitution reaction of cinnamyl (=3‐phenylprop‐2‐enyl) phosphate 12 with diethylzinc as a nucleophile. Ligands 5ae, 6ae, 8ae , and 11ae gave excellent ratios (100 : 0) of the S_N2′/S_N2 products (Scheme 6 and Table 1). Ligand 11ce , identified from the screening of a small library of ligands of general formula 11 , promoted the allylic substitution reaction with moderate enantioselectivity (40% for the S_N2′ product 13 (Scheme 8 and Table 3)).     

New beta-lactam antibiotics. Cephem derivatives with electron withdrawing substituents at position 3 (author's transl)]

New β-lactam antibiotics. Cephem derivatives with electron withdrawing substituents at position 3 Oxidation of the 3-formyl-2-cephem compound 1 according to Corey [6] gave 2-cephem-3-carboxylic esters 4a, b, c (Scheme 1), which proved to be useful intermediates for the synthesis of cephalosporins bearing in position 3 a methoxycarbonyl group (10a, b, c, d / Scheme 2) or a carboxy group (20, 25, 30/Schemes 3, 4). The 3-formyl-3-cephem compounds 31a, b could be transformed into cyano- (33a) or methoxyiminomethyl- (36a, c, d) cephems (Scheme 5), which represent further examples of cephalosporins with electron withdrawing groups in position 3.     

An Unexpected Formation of a 14‐Membered Cyclodepsipeptide

The treatment of diluted solutions of the hydroxy diamides 6a and 6b in toluene with HCl gas at 100° gave the dimeric, 14‐membered cyclodepsipeptide 10 in up to 72% yield (Scheme 3). The same product was formed from the linear dimer of 6b , the depsipeptide 11 , under the same conditions (cf. Scheme 4). All attempts to prepare the cyclic seven‐membered monomer 9 , starting with different precursors and using different lactonization methods failed, and 10 was the only product which was isolated (cf. Scheme 6). For example, the reaction of the ester 20 with NaH in toluene at 80° led exclusively to the cyclodimer 10 . On the other hand, the base‐catalyzed cyclization of the hydroxy diester 22 , which is the ‘O‐analogue' of 20 , yielded neither the seven‐membered dilactone, nor the 14‐membered tetralactone, but only the known trimer 23 and tetramer 24 of 2,2‐dimethylpropano‐3‐lactone (cf. Scheme 7).     

Determination of the photochemical efficiency of o-quinodimethane ring closure in room-temperature solutions by using time-delayed, two-color photolysis technique

Photochemical efficiency of o-quinodimethane (3) ring closure at room temperature was determined by using a time-delayed, two-color photolysis technique. o-Quinodimethane (3) was generated by the photolysis of 1,2-bis[(phenylseleno)methyl]benzene (1) by a KrF (248 nm) laser pulse and thus-generated 3 was photolyzed by a subsequent XeCl (308 nm)/XeF (351 nm) laser pulse with varying delay time of 0 to 3 s. The time profile of 3 was monitored by the chemical analyses of benzocyclobutene (5) (a photochemical product of 3), which was formed by a one-photon process, and the spiro dimer of 3 (4) (a thermal product of 3) in the two-color photolysis experiments. The time profile of 3 followed a second-order decay kinetics. The photochemical efficiency was obtained by the analysis of the delay-time dependence of the product yields; those of the consumption of 3 and the conversion 3-->5 by a single pulse of the excimer laser were 81% and 5.7% for the XeCl laser, and 73% and 2.3% for the XeF laser. This difference was attributed to the different excited states involved in the photolysis. In contrast to the photolysis of 3 in argon or rigid organic matrixes, it was revealed that photochemical conversion 3-->5 was not the main path in the solutions, and intermolecular reactions predominated.     

1,5-Dipolare Elektrocyclisierung von Acyl-substituierten ‘Thiocarbonyl-yliden’ zu 1,3-Oxathiolen

1,5-Dipolar Electrocyclization of Acyl-Substituted ‘Thiocarbonyl-ylides’ to 1,3-Oxathioles The reaction of α-diazoketones 15a, b with 4,4-disubstituted 1,3-thiazole-5(4H)-thiones 6 (Scheme 3), adamantanethione ( 17 ), 2,2,4,4-tetramethyl-3-thioxocyclobutanone ( 19 ; Scheme 4), and thiobenzophenone ( 22 ; Scheme 5), respectively, at 50–90° gave the corresponding 1,3-oxathiole derivatives as the sole products in high yields. This reaction opens a convenient access to this type of five-membered heterocycles. The structures of three of the products, namely 16c, 16f , and 20b , were established by X-ray crystallography. The key-step of the proposed reaction mechanism is a 1,5-dipolar electrocyclization of an acyl-substituted ‘thiocarbonyl-ylide’ (cf. Scheme 6). The analogous reaction of 15a, b with 9H-xanthen-9-thione ( 24a ) and 9H-thioxanthen-9-thione ( 24b ) yielded α,β-unsaturated ketones of type 25 (Scheme 5). The structures of 25a and 25c were also established by X-ray crystallography. The formation of 25 proceeds via a 1,3-dipolar electrocyclization to a thiirane intermediate (Scheme 6) and desulfurization. From the reaction of 15a with 24b in THF at 50°, the intermediate 26 (Scheme 5) was isolated. In the crude mixtures of the reactions of 15a with 17 and 19 , a minor product containing a CHO group was observed by IR and NMR spectroscopy. In the case of 19 , this side product could be isolated and was characterized by X-ray crystallography to be 21 (Scheme 4). It was shown that 21 is formed – in relatively low yield – from 20a . Formally, the transformation is an oxidative cleavage of the C?C bond, but the reaction mechanism is still not known.     

Eine neue Synthese von (±)-Dihydrorecifeiolid

A New Synthesis of (±)-Dihydrorecifeiolide Ethyl 1-(2′-formylethyl)-2-oxocyclooctane-1-carboxylate ( 2 ) prepared by Michael reaction of ethyl 2-oxocyclooctane-1-carboxylate ( 1 ) was regioselectively methylated at the aldehyde group with (CH₃)₂Ti[OCH(CH₃)₂]₂ to give 3 (Scheme 1). The alcohol 3 was treated with Bu₄NF to give the deethoxycarbonylated product 4 which by distillation gave the bicyclic enol ether 5 . Oxidation (m-chloroperbenzoic acid) of 5 and reduction of the resulting oxolacton 6 yielded the title compound (±)-dihydrorecifeiolide ( 7 ) in an overall yield of nearly 50 %. Methylation of the aldehyde 2 with MeLi gave the ring-enlarged lacton 9 in poor yield (13 %). The deethoxycarbonylation reaction 3 → 4 was studied in more detail (Scheme 3).