1,3-Dipolar cycloaddition of nitrones with α,β-unsaturated sulfones

C,N-Diphenyl nitrones react with substituted α,β-unsaturated phenylsulfones yielding isoxazolidinic cycloadducts whose structure and stereochemistry were assigned on the basis of ¹H and ¹³C nmr data. The ycloaddition regioselectivity is discussed in accordance with frontier orbital considerations.     

Isomerization of unsaturated radicals. V. Unimolecular isomerization of hot α,α- and α,β-dimethallyl radicals

The 147 nm photolysis of 3,3 dimethylbut-1-ene leads mainly to the formation of very hot (?375 kJ/mol) α,α-dimethallyl radicals. On the other hand, that of 3-methyl-cis-and trans-pentene-2, as well as that of 2,3-dimethylbut-1-ene is a source of very hot α,β-dimethallyl radicals. These allylic radicals are coolled down using pressure and are allowed to combine with available methyl radicals. From the formation of various C₆H₁₂ products, it is concluded that the very hot α,α- radical isomerizes towards the α,β-structure at low pressures and vice versa. The equilibrium constant of the following process has been evaluated to be 1.72 ± 0.30.      

4,5-Dihydroisoxazole and 4,5-dihydro-1,2,4-oxadiazole derivatives from cycloaddition reactions of nitrile oxides to alkyl N-(diphenylmethylene)-α,β-dehydroamino acids

The alkyl N-(diphenylmethylene)-α,β-dehydroamino acids 1 have been submitted to 1,3-dipolar cycloadditions with nitrile oxides. The reactivity of these compounds depends on the type and on the stereochemistry of the β-substituents. With the unsubstituted terms 1a,b the reaction occurs on the C,C double bond, providing a good method for the synthesis of the 4,5-dihydroisoxazole derivatives 3a,b,c and for the gem-functionalized 4,5-dihydroisoxazoles amino carboxylic ester 5. The β-substituted compounds 1c,d,e , inert to 1,1-dimethylethylnitrile oxide, undergo the reaction to the N,C double bond, thus giving with 2a,b the 4,5-dihydro-1,2,4-oxadiazole derivatives 4. All the reactions occur with high site- and regioselectivity.     

Diastereoselectivity and Reactivity in the Diels-Alder Reactions of α-Chloronitroso Ethers

The structure of the α-chloronitroso ether 1 , obtained from the hydroximolactone 2 and tert-butyl hypochlorite (89%), was established by X-ray crystallographic analysis. The [4 + 2] cycloadditions of 1 with the dienes 3 and 8 – 11 led to the N-unsubstituted 3,6-dihydro-2H-1,2-oxazines 6 and 12 – 16 in high enantiomeric excess (Table 1). Due to the additional α-alkoxy group, the reactivity of 2 is much superior to the one of known α-chloronitrosoalkanes. The reactive conformation of 1 was deduced from the X-ray analysis as well as the high diastereoselectivity of the cycloadditions. The importance of the α-alkoxy group was evidenced from the similar reactivity of the racemic α-chloronitroso ethers 25 – 27 which were prepared from the hydroximo ethers 28 – 30 and tert-butyl hypochlorite.     

Über 9β, 10α-Steroide. 2. Mitteilung. Strukturaufklärung von mikrobiologischen Umsetzungsprodukten mit Sauerstoff-Funktion in Stellung 11

The structure and stereochemistry of some 11α- and 11β-hydroxy-9β, 10α(retro)-steroids of the androstane, pregnane and corticoid series, obtained microbiologically, has been elucidated.     

Synthesis and transformations of (±)-trans-4aβ(H), saα(H)-octahydro-4α,7β-dimethyl-2H-1-benzopyran-2-one

Hydrogenation of 4,7-dimethylcoumarin ( 1 ) in alkaline medium has been shown to furnish a mixture of (±)-trans-4aβ(H),8aα(H)-octahydro-4α,7β-dimethyl-2H-1-benzopyran-2-one ( 2 ), (±)-trans-4aβ(H),8aα(H)-octahydro-4α,7α-dimethyl-2H-1-benzopyran-2-one ( 3 ) and (±)-cis-4aα(H),8aα(H)-octahydro-4α,7α-dimethyl-2H-1-benzopyran-2-one ( 4 ) in 40:25:35:ratio, respectively. The stereochemistry of the major hydrogenation product 2 , has been established by transforming it to p-menthane derivatives e.g. (±)-2 (R)-[2′(R)hydroxy-4′(R) methylcyclohex-(1′S)-yl]propan-1-ol ( 20 ) and (±)-trans-3α,6β-dimethyl-3aβ(H),7aα(H)-octahydrobenzofuran ( 12 ). Starting from a mixture of lactones 2, 3 and 4 , lactone 3 has been obtained in pure state employing a sequence of reactions.     

Schwermetallchelate von α-Cyano-β-amino-dithioacrylsäureestern

Heavy Metal Chelates of α-Cyano-β-amino-dithioacrylates The inner metal chelates of Pd^II, Ni^II, Co^III, and Ag^I with α-cyano-β-amino-dithioacrylates have been prepared. The coordination of the dithioester group and the amine nitrogen (S/N-coordination) has been proved by the chemical shift of the S2p and N1s electron binding energies in the ESCA spectra.     

Cob (I)alamin als Katalysator. 3. Mitteilung [1]. Untersuchungen in Richtung einer enantioselektiven Reduktion α,β-ungesättigter Ester

Cob (I)alamin as Catalyst 3. Communication [1]. Examination of an Enantioselective Reduction Using α,β-Unsaturated Esters α,β-Unsaturated esters can be reduced to the corresponding saturated esters using catalytic amounts of cob (I)alamin in the presence of an excess of zinc. An enantioselective reduction has been observed starting from ethyl (Z)-3-methyl-5-phenyl-2-pentenoate (7).     

GC-MS investigation of α-polyfluoroalkylbenzyl phosphorus esters

The stereochemistry of some α-polyfluoroalkylbenzyl phosphorus esters has been studied by capillary gas chromatography (CGC). The compounds are mixtures of two or three diastereomers, depending on the number of asymmetric centers and the molecular symmetry. The primary fragmentation processes in the mass spectra of the phosphorus esters and their deuterated analogous have also been studied.     

Lipophilic Functionalized Cobyrinic-Acid Derivatives. Part 1. Cobester α-monoacids (α,α,α,β,β,β-hexamethyl α-hydrogen Coα, Coβ-dicyanocobyrinates)

The four α,α,α, β,β,β,-hexamethyl α-hydrogen Coα, Coβ-dicyanocobyrinates 2b, d–f , with a free b-, d-, e-, and f-propionic-acid function, respectively, were prepared by partial hydrolysis of heptamethyl Coα, Coβ-dicyanocobyrinate (cobester; 1 ) in aqueous sulfuric acid. The cobester monoacids 2b, d–f were obtained as a ca. 1:1:1:1 mixture which was separated. The monoacids were purified by chromatography and isolated in crystalline form. The position of the free propionic-acid function was determined by an extensive analysis of 2b, d–f using 2D-NMR techniques; an analysis of the C,H-coupling network topology resulted in an alternative assignment strategy for cobyrinic-acid derivatives, based on pattern recognition. Additional information on the structure of the most polar of the four hexamethyl cobyrinates, of the b-isomer 2b , was also obtained in the solid state from a single-crystal X-ray analysis. Earlier structural assignments based on 1D-NMR spectra of the corresponding regioisomeric monoamides 3b, d–f (obtained from crystalline samples of the monoacids 2b, d–f ) were confirmed by the present investigations.     

Photochemische Reaktionen. 82. Mitteilung. Zur Photochemie der α,β-Epoxylketone: γ-Wasserstoffabstraktion versus Epoxyketon-Umlagerung

Photochemistry of α,β-epoxyketones: γ-H-abstraction versus epoxyketone-rearrangement. The photochemical behaviour of conformationally mobile α,β-epoxyketones that could undergo competing reactions has been studied. The UV.-irradiation of 5 yields the stereoisomeric cyclobutanols 9 and 10 as well as the fragmentation product 11 . Irradiation of 6 gives only the cyclobutanols 12 and 13 , whereas 4 and 8 in inert solvents yielded only intractable gums. The non-occurrence of the typical isomerization of α,β-epoxylketones to the corresponding β-diketones is attributed to steric factors.     

Applications of 1,4-addition of grignard reagents to α,β-unsaturated acid derivatives. III—configuration studies of α,β-diaryl-α,β-dialkylpropionitriles

The configuration of twenty-four different α,β-diaryl-α,β-dialkylpropionitriles has been determined using nuclear magnetic resonance spectra. The erythro form showed greater deshielding of the methine hydrogen and alkyl groups in its spectra than the corresponding threo isomer. The former, however, showed less deshielding of the aryl groups than the threo isomer. Infrared spectra were not suitable for assignments of configuration.     

Studies in organic mass spectrometry-XIII: Investigation of electron-induced isomerisation of αβ- and βγ-unsaturated esters

The mass spectral fragmentation of methyl esters of α,β- and β,γ-unsaturated carboxylic acids has been studied. Deuterium labelling, metastable ion analysis and high resolution mass spectrometry have been utilised to elucidate the mechanism of a number of fragmentations and to check possible double bond migration prior to fragmentation. Some breakdown modes were found to occur through double bond migration. Additional support was provided by application of the ‘Metastable Ion Characteristics’ method. Partial isomerisation of molecular ions must therefore be accepted.     

Annelation of α,β-unsaturated esters to thian-3-one 1-oxides using dianion of ω-(methylsulfinyl)acetophenone

β-Monosubstituted α,β-unsaturated esters undergo an annelation with dilithio salt of ω-(methylsulfinyl)-acetophenone to produce 5-substituted 6-benzoylthian-3-one 1-oxides.     

Über 9β,10α-Steroide. 4. Mitteilung [1]. Strukturaufklärung von mikrobiologischen Umsetzungsprodukten

The structure and the stereochemistry of some 8- and 9-hydroxy-9β, 10° (retro)-steroids of the androstane and pregnane series, obtained microbiologically, have been elucidated.     

Säurekatalysierte Isomerisierung von α,β-Cyclopropyl-oxiranen

Using the epoxycarane derivatives 3 and 6 , the behaviour of α,β-epoxycyclopropy. compounds in the presence of acids has been examined and the main reaction products elucidated Proposals for the reaction mechanism are made.     

Stereoselective Aldol Reactions with α-Unsubstituted Chiral Enolates

The aldol reaction is among the most important methods of forming carbon-carbon bonds. The addition of an enolate to an aldehyde leads to the formation of at least one chiral center. In the case of α-substituted enolates it has to a large extent been possible to control the product stereochemistry, while the aldol reaction of α-unsubstituted chiral enolates was for many years a “problem child” for synthetic chemists because of its insufficient stereoselectivity. Progress in this area has only been made in the last few years using either new chiral auxiliaries or alternatives to the aldol reaction.     

Reactions of β-heteroatom substituted α,β-unsaturated ketones with dimethylsulfonium methylide

β-Alkoxy- and β-alkylthio α,β-unsaturated ketones react with dimethylsulfonium methylide (DMSM) to give a furan, while β-anilino α,β-unsaturated ketones give the pyrrole. β,β-Bis(alkylthio) α,β-unsaturated ketones react with DMSM to afford the methylene inserted products, λ,λ-bis(alkylthio) β,λ-unsaturated ketones.     

Carbon-13 chemical shifts of α,β-unsaturated acids

Carbon-13 spectra of 59 α,β-unsaturated carboxylic acids have been measured. Large differential shieldings of ethylenic carbons in the cis and trans isomers of acrylic acid derivatives were found (altogether 15 isomeric Z- and E-pairs of acids were investigated). The origin of differential shieldings is complex and both molecular ground states as well as changes in excited states appear to be involved. All measured ¹³C chemical shifts can be described by additive parameters that provide a straightforward new technique by which structural assignments can be made for a wide variety of isomeric mono-, di- and tri- substituted α,β-unsaturated acids.     

Synthesis of α-hydroxy-β-lactams

A convenient synthesis of α-hydroxy-β-lactams has been devised that involves the annelation of an inline with benzyloxyacetyl chloride and triethylamine and subsequent hydrogenolysis in the presence of palladium on carbon. In most cases a cis-β-lactam was obtained. A thioimidate can also be used as the imino component in the annelation reaction but the hydrogenolysis step fails. The annelation of the appropriate thiazoline to a 6-epi-penicillin derivative occurred much more readily with benzyloxyacetyl chloride than with azidoacetyl chloride.