Amino-zinc-enolate carbometalation reactions: application to ring closure of terminally substituted olefin for the asymmetric synthesis of cis- and trans-3-prolinoleucine

The amino-zinc-enolate cyclization reaction is a straightforward route for the synthesis of 3-substituted prolines. As classical intramolecular carbometalation reactions, the applicability of the addition of zinc to a double bond was limited to a substrate in which the terminal alkene carbon was unsubstituted. Being interested in the synthesis of cis- and trans-3-prolinoleucine derivatives for our structure-activity relation (SAR) studies, we focused our effort on the preparation of these compounds by amino-zinc-enolate cyclization of terminally substituted double bonds. Herein we report that the attachment of an activating group such as cyclopropyl to the terminal olefin carbon allows the amino-zinc-enolate cyclization of a terminally substituted alkene. The reaction is stereospecific, leading to a trans-3-substituted proline derivative, whereas a cis stereochemistry was observed with the amino-zinc-enolate cyclization of terminally nonsubstituted olefins. Absolute configurations obtained for the 3-prolinoleucine were established by X-ray analysis, NMR, and optical activity comparison of the cis and trans derivatives obtained by an unambiguous pathway.     

Intramolecular Radical,Knoevenagel, or SN2′ Cyclization of Carbohydrate Derivatives for Access to Enantiomerically Pure 2-Oxospiroalkanes

Abstract

Intramolecular radical cyclization of D-glucose-derived substrate, (2R,3R,4R,5S)-2, 3-(isopropylidenedioxy)-5-[(1R)-l, 2-(isopropylidenedioxy)ethyl]-4-[3-bromo-3, 3-bis(methoxycarbonyl)propyl]-4-vinyltetrahydrofuran, 7 proceeded in a 6-endo-trig mode to give a derivative of 2-oxaspiro[4.5]decane 8 exclusively. Intramolecular Knoevenagel-like reaction of substrate 9 afforded derivatives of 2-oxaspiro-[4.4]nonane 10 as a 3:1 diastereomeric mixture. Intramolecular SN²′ displacement of substrate 22 proceeded highly stereoselectively giving a derivative of 2-oxaspiro-[4.4]decane 23.     

Super acid-induced Pummerer-type cyclization reaction: improvement in the synthesis of chiral 1,3-dimethyl-1,2,3,4-tetrahydroisoquinolines

Improved synthesis of four stereoisomeric chiral 1,3-dimethyl-1,2,3,4-tetrahydroisoquinolines (1a, b, ent-1a, b) was achieved via the super acid-induced cyclization of chiral N-[1-methyl-2-(phenylsulfinyl)ethyl]-N-(1-phenylethyl)formamides (4a, b, ent-4a, b) using the Pummerer-type cyclization reaction as a key step. The cyclization leading to the isoquinoline ring proceeded in a quantitative manner when trifluoromethane sulfonic acid (TFSA) was used as the super acid, although Friedel-Crafts-type alkylation of 4-phenylsulfanyl TIQ derivatives (5) with benzene used as the solvent accompanied cyclization to yield the 4-phenyl-TIQs (7). The byproduct (7) was exclusively formed when a large excess amount of TFSA was used.     

A Lewis acid-promoted cyclization of ethenetricarboxylate derivative aromatic compounds. Novel syntheses of oxindoles and benzofuranones via Friedel-Crafts intramolecular Michael addition

A novel cyclization reaction of ethenetricarboxylate derivative aromatic compounds in the presence of various Lewis acids gave benzo-annulated cyclic compounds such as oxindole and benzofuran derivatives via Friedel-Crafts intramolecular Michael addition in high yields. For example, the reaction of diethyl 2-[(N-methyl-N-phenylcarbamoyl)methylene]malonate (1a) in the presence of ZnCl2 at room temperature gave diethyl 2-(1-methyl-2-oxoindolin-3-yl)malonate (2a) in 98% yield. The reactions also proceeded with a catalytic amount of a Lewis acid such as AlCl3, ZnCl2, ZnBr2, Sc(OTf)3, or InBr3.     

Synthetic investigations of (1,3′)-bistetrahydroisoquinolines: towards pentacyclic analogues of piperazine core alkaloids

Synthetic investigations of (1,3′)-bistetrahydroisoquinolines are reported as the key intermediates for the synthesis of ecteinascidin and phthalascidin pentacyclic structure analogues through successive Pictet-Spengler cyclization and intramolecular peptide coupling. The direct Pictet-Spengler reaction between a derivative of l-DOPA and N-protected-α-aminoaldehyde was first extended to the synthesis of cis-(1,3′)-bistetrahydroisoquinoline. After introduction of the required amino acid moiety, an efficient six-membered ring intramolecular peptide coupling gave rise to piperazine derivative structures. Complete structural assignments were corroborated by NMR and X-ray spectroscopic methods. Nevertheless, the optical integrity of the N-protected-α-aminoaldehyde seems to be sensitive to the reaction conditions. Pentacylic structures, having an anti C3-C11 backbone stereochemistry, were obtained from cyclization para- and ortho- to the 3-OH group of the l-DOPA derivative.     

Intramolecular Nicholas reaction: stereoselective synthesis of 5-alkynylproline derivatives

The intramolecular Nicholas reaction of propargylic alcohols derived from N, N-acyl-diprotected omega-semialdehydes obtained from glutamic acid provided stereoselectively 5-alkynylproline derivatives. The suitable choice of the N-protecting group (tosyl or benzoyl derivative) permitted control of the stereochemistry during the ring formation. Semiempirical calculations of the species involved in the cyclization support the observed stereochemistry.     

Stereoselectivity of intramolecular SN' cyclizations of alkyllithium reagents on methoxy alkenes

The cyclization of alkyllithium reagents onto methoxy alkenes has been investigated. The alkyllithium reagent was generated by reductive lithiation of an alkyl nitrile. In an unbiased substrate, a methoxy leaving group attached to a stereogenic secondary carbon atom led to the cyclization product with high optical purity. The configuration of the product demonstrated that the cyclization had proceeded with high syn-S(N)' selectivity. Previously we have shown that 2-lithiotetrahydropyran reagents cyclize to form spirocycles with the alkene cis to the pyran oxygen. Substrates were prepared to evaluate the importance of the configuration of the secondary allyl methyl ether against the alpha-alkoxy alkyllithium configuration. In the matched case (cyano acetal 38), a very selective cyclization ensued. In the mismatched case (cyano acetal 39), the spiro ether selectivity dominated. The syn-S(N)' selectivity overcame the normal E selectivity in the cyclization and accounted for the major product, Z-alkene 45. Thus the stereochemical preference in these alkyllithium cyclizations is dominated by the spiroether effect, followed by the syn-S(N)' selectivity and finally the preference for E-alkene formation.     

Total synthesis of a cytotoxic acetogenin,pyranicin

[structure: see text] The first total synthesis of a new cytotoxic acetogenin, pyranicin (1), is described. SmI(2)-induced reductive cyclization of beta-alkoxy acrylate 4 proceeded stereoselectively to give 16,20-syn-19,20-trans-THP derivative 14, which was efficiently transformed into the 19,20-cis-THP derivative 18 through Mitsunobu lactonization. Wittig reaction of the phosphonium salt 2 obtained therefrom with butenolide 3 at -78 degrees C followed by reduction and deprotection afforded 1 in good overall yield.     

Carbocyclization reaction of omega-iodo- and 1,omega-diiodo-1-alkynes without the loss of iodine atoms through a carbenoid-chain process

Atom-economical carbocyclization reactions of omega-iodo-1-alkynes and 1,omega-diiodo-1-alkynes to give products with incorporation of iodine atoms is described. Cycloisomerization of 2-(2-propynyloxy)ethyl iodides is initiated by a catalytic amount of LDA to give 3-(iodomethylene)tetrahydrofurans in high yields. Upon treatment of with a catalytic amount of 1-hexynyllithium, 1,omega-diiodo-1-alkynes efficiently undergo cycloisomerization to give (diiodomethylene)cycloalkanes. The diiodomethylene products are also obtained by iodine atom-transfer-type cyclization of omega-iodo-1-alkynes, using 1-iodo-1-hexyne as an external iodine atom source. Bromine atom-transfer and proton-transfer cyclization proceed as well by employing 1-bromo-1-octyne and 1-octyne, respectively. These reactions are proposed to proceed through a carbenoid-chain process involving exo-cyclization of the lithium acetylide intermediates to give Li,I-alkylidene carbenoids. It is shown that the exo-cyclization proceeded stereospecifically through inversion of the stereochemistry at the electrophilic carbon.     

Comparative study of anionic and radical cyclization for the preparation of 1,3-dimethylindans: highly stereoselective preparation of cis-1,3-disubstituted indans via intramolecular carbolithiation

[reaction: see text] The preparation of 1,3-dimethylindans from 4-(2-bromophenyl)-1-pentene (1) and 2-(2-iodo-1-methylethyl)styrene (2) substrates via radical-mediated cyclization and intramolecular carbolithiation has been investigated. Although cyclization of the radical derived from either substrate proceeds with modest selectivity for the cis-isomer, as does cycloisomerization of the aryllithium derived from substrate 1 (cis/trans approximately 2), intramolecular cyclization of the alkyllithium derived from substrate 2 is a highly cis-selective process (cis/trans = 12).     

Inversion of Configuration at the Phosphorus Nucleophile in the Diastereoselective and Enantioselective Synthesis of P‐Stereogenic syn‐Phosphiranes from Chiral Epoxides

Nucleophilic substitution results in inversion of configuration at the electrophilic carbon center (S_N2) or racemization (S_N1). The stereochemistry of the nucleophile is rarely considered, but phosphines, which have a high barrier to pyramidal inversion, attack electrophiles with retention of configuration at P. Surprisingly, cyclization of bifunctional secondary phosphine alkyl tosylates proceeded under mild conditions with inversion of configuration at the nucleophile to yield P‐stereogenic syn‐phosphiranes. DFT studies suggested that the novel stereochemistry results from acid‐promoted tosylate dissociation to yield an intermediate phosphenium‐bridged cation, which undergoes syn‐selective cyclization.     

Extending Pummerer reaction chemistry. development of a strategy for the regio- and stereoselective oxidative cyclization of 3-(omega-nucleophile)-tethered indoles

The brominative cyclization of diastereomeric beta-silyloxy tryptophan derivatives proceeded with divergent regiochemistry (C(2) or C(3) addition), depending on the relative stereochemistry of the silyloxy substituent. This lack of C(2) vs C(3) regiochemical predictability led to the development of a new approach, which featured Pummerer-type chemistry on an indole C(2) sulfoxide or sulfide substrate, for steering nucleophilic addition to C(3) of the indole. Extension of this transformation from carboxylate nucleophiles to carbon analogues such as allylsilane, silyl enol ether, and silyl ketene iminal bearing substrates led to the formation of spirocyclic oxindole derivatives in good yields with complete regioselectivity for C(3) cyclization and with good diastereoselectivity where relevant.     

Anomalous substituent effects in the bischler-napieralski reaction of 2-aryl aromatic formamides

Treatment of some 1-naphthylformamides (or formanilides) possessing a 2,4,5-trioxygenated phenyl substituent at the 2-position with POCl(3) caused an unprecedented carbon insertion reaction into a benzene ring, producing 7-5 ring (azaazulene) systems as valence isomers of isoquinoline skeletons. Precise examination of this abnormal Bischler-Napieralski reaction (BNR) using various substrates led to the following scope and limitations: (i) the 7-5 ring systems were constructed when either 2-alkoxy-4, 5-methylenedioxyphenyl- or 4,5-dialkoxy-2-hydroxyphenyl-substituted formamides were used as a starting substrate; (ii) in the former case the formyl carbon was inserted into the C(1)-C(6) bond of the 2-phenyl group, and normal isoquinoline cyclization competed with an abnormal carbon insertion reaction; (iii) the presence of a hydroxy group at the 2'-position as in the latter cases caused exclusive carbon insertion, in which alternative C(1)-C(2) insertion products were quantitatively formed; (iv) 3, 6-dimethoxy-2-hydroxyphenyl-substituted formanilide electronically equivalent to 4,5-dialkoxy-2-hydroxy derivatives produced an indole-pyrone as an abnormal BNR product. Theoretical approaches using the PM-3 method indicated that these abnormal BNRs could be triggered by ipso attack at the 1'-position yielding spiro intermediates. Ring cleavege of the six-membered ring in the spiro intermediates to a ketene function followed by recyclization was proposed for the 2'-hydroxy-directed abnormal BNRs leading to the C(1)-C(2) insertion product or the indole-pyrone derivative.     

Pyrrolo[1,4]benzodiazepines. III. synthesis of 5,11-dioxo-1,10,11,11a-tetrahydro-2-vinyl-5H-pyrrolo[2,1-c][1,4]benzodiazepine

Some pyrrolo[2,1-c][1,4]benzodiazepines having a chain of two carbon atoms, as it is found in antibiotic tomaymycin, were prepared. The first reaction was the C-acetylation of 1-(2-nitrobenzoyl)-Δ⁴-pyrroline-2,2-dicarboxylic acid diethyl ester; successive transformations of the acetyl group and cyclization gave the proposed structures.     

The synthesis of (-)-isodomoic acid C

The neuroactive algal metabolite (-)-isodomoic acid C, a kainoid amino acid, has been synthesized for the first time. Asymmetric dearomatizing cyclization of an aromatic amide using a chiral lithium amide base generates a bicyclic enone containing a pyrrolidinone ring with the relative and absolute stereochemistry of the target. A further 15 synthetic steps, including conjugate cuprate addition to the enone of a side chain precursor, a Ru-promoted oxidation of the phenyl ring to the C2-carboxylic acid substituent, a regioselective Baeyer-Villiger reaction, and an E-selective Horner-Wadsworth-Emmons reaction, elaborate the cyclization product into the target molecule.     

A synthesis of 3-phenyl-1,2,3,4-tetrahydroisoquinoline and 2-phenyl-1,2,4,5-tetrahydro-3H-3-benzazepine via Pummerer-type cyclization: enhancing effect of boron trifluoride diethyl etherate on the cyclization

A synthesis of 6,7-dimethoxy-3-phenyl-1,2,3,4-tetrahydroisoquinoline (14a) and 7,8-dimethoxy-2-phenyl-1,2,4,5-tetrahydro-3H-3-benzazepine (14b) was achieved via the cyclization of N-(3,4-dimethoxyphenyl)methyl-1-phenyl-2-(phenylsulfinyl)ethylformamide (6a) and N-2-(3,4-dimethoxyphenyl)ethyl-1-phenyl-2-(phenylsulfinyl)-ethylformamide (6b) using the Pummerer reaction as a key step, respectively. The Pummerer reaction of 6a,b under usual conditions using trifluoroacetic anhydride yielded the vinyl sulfides (8a, b), non-cyclized products, as a major product. The cyclization proceeded when boron trifluoride diethyl etherate was used as an additive reagent, thus giving rise to the corresponding cyclized products (7a) and (7b) in moderate yields. We propose that the enhancing effect of the Lewis acid on the cyclization may be attributable to the involvement of a dicationic intermediate, sulfonium-carbenium dication (23).     

Total synthesis of prostaglandin F2alpha using nickel-catalyzed stereoselective cyclization of 1,3-diene and tethered aldehyde via transmetalation of nickelacycle with diisobutylaluminum acetylacetonate

Total synthesis of prostaglandin F2alpha utilizing a nickel(0)-catalyzed cyclization of 1,3-diene and tethered aldehyde was achieved. The cyclization proceeded via a transmetalation of nickelacycle with diisobutylaluminum acetylacetonate (iBu2-ALAC). Thus, the reaction of 19, having a side chain corresponding to the alpha-chain in PGF2alpha with Ni(cod)2 (10 mol %), PPh3 (20 mol %), and 1,3-cyclohexadiene (25 mol %) in the presence of iBu2-ALAC (1.5 eq) proceeded stereoselectively to give the cyclized product 26 in 54% yield. During the cyclization of 19, the Z-olefin at C-5 in the side chain completely retained its geometry, and the four contiguous chiral carbon centers in PGF2alpha were stereoselectively constructed. Transformation of the key intermediate 19 into PGF2alpha was successfully achieved.     

Stereoselective synthesis of 2-alkylidene-3-iminoindoles by reaction of 1,1-dianions with oxalic acid bis(imidoyl) chlorides

Treatment of dilithiated nitriles and sulfones with oxalic acid bis(imidoyl) chlorides resulted in a new cyclization reaction which provided a variety of (3-imino-2, 3-dihydro-1H-indol-2-ylidene)acetonitriles and -sulfones in good yields. The reactions proceeded by condensation of the dianions with the first imidoyl chloride group of the bis(imidoyl) chloride, subsequent intramolecular attack of the ortho carbon of the arylimino group onto the second imidoyl chloride group, and final aromatization. Excellent stereoselectivities were observed in most cases.     

Total synthesis of the marine alkaloids (-)-lepadins a, b, and c based on stereocontrolled intramolecular acylnitroso-Diels-Alder reaction

The first syntheses of (-)-lepadins A and C, as well as a new synthesis of (-)-lepadin B, have been achieved from commercially available (S)-malic acid. The methodology is based on an intramolecular hetero-Diels--Alder reaction of the acylnitroso compound, affording the bicyclic oxazino lactam with trans selectivity, which was converted to the cis-decahydroquinoline via asymmetric enolate hydroxylation followed by intramolecular aldol cyclization. The total syntheses proceed by employing cis-decahydroquinoline bearing the (E)-iodoalkenyl group as the common key intermediate, which underwent a convergent coupling with the (E)-hexenyl unit via a palladium-catalyzed Suzuki cross-coupling reaction for the elaboration of the octadienyl side chain at the C5 position.     

Enzymatic formation of an unnatural hexacyclic C35 polyprenoid by bacterial squalene cyclase

A C35 polyprene in which a farnesyl C15 unit is connected in a head-to-head fashion to a geranylgeranyl C20 unit was enzymatically converted to an unnatural hexacyclic polyprenoid by squalene:hopene cyclase from Alicyclobacillus acidocaldarius. This is the first demonstration of the remarkable ability of the squalene cyclizing enzyme to perform construction of unnatural hexacyclic skeleton. The cyclization of the C35 polyprene was initiated by a proton attack on the terminal double bond of the C15 unit and proceeded without rearrangement of carbon and hydrogen. The substrate should be folded in chair-chair-chair-chair-boat-boat conformation to achieve the stereochemistry of the cyclization product.