Kinetic resolution and parallel kinetic resolution of methyl (+/-)-5-alkyl-cyclopentene-1-carboxylates for the asymmetric synthesis of 5-alkyl-cispentacin derivatives

Conjugate addition of lithium dibenzylamide to methyl 5-isopropyl, 5-phenyl- and 5-tert-butyl-cyclopentene-1-carboxylates occurs with high levels of substrate control (>88% de), with preferential addition to the face of the cyclic alpha,beta-unsaturated acceptor anti- to the stereodirecting 5-alkyl substituent. Treatment of a range of methyl (+/-)-5-alkyl-cyclopentene-1-carboxylates with both lithium (+/-)-N-benzyl-N-alpha-methylbenzylamide and lithium (+/-)-N-3,4-dimethoxybenzyl-N-alpha-methylbenzylamide indicates significant enantiorecognition in their mutual kinetic resolutions, with preferential addition anti- to the 5-alkyl substituent, giving the 1,2-syn-1,5-anti-arrangement (E >16) after enolate protonation anti- to the amino functionality. The kinetic resolution of a range of methyl (+/-)-5-alkyl-cyclopentene-1-carboxylates with lithium (S)-N-benzyl-N-alpha-methylbenzylamide, and their efficient parallel kinetic resolution with a pseudoenantiomeric mixture of lithium (S)-N-benzyl-N-alpha-methylbenzylamide and lithium (R)-N-3,4-dimethoxybenzyl-N-alpha-methylbenzylamide are also demonstrated, giving a range of 5-alkyl-cispentacin derivatives in >98% de and high ee after N-deprotection.     

Asymmetric synthesis of (1R,2S,3R)-3-methylcispentacin and (1S,2S,3R)-3-methyltranspentacin by kinetic resolution of tert-butyl (+/-)-3-methylcyclopentene-1-carboxylate

Conjugate addition of lithium dibenzylamide to tert-butyl (+/-)-3-methylcyclopentene-1-carboxylate occurs with high levels of stereocontrol, with preferential addition of lithium dibenzylamide to the face of the cyclic alpha,beta-unsaturated acceptor anti- to the 3-methyl substituent. High levels of enantiorecognition are observed between tert-butyl (+/-)-3-methylcyclopentene-1-carboxylate and an excess of lithium (+/-)-N-benzyl-N-alpha-methylbenzylamide (10 eq.) (E > 140) in their mutual kinetic resolution, while the kinetic resolution of tert-butyl (+/-)-3-methylcyclopentene-1-carboxylate with lithium (S)-N-benzyl-N-alpha-methylbenzylamide proceeds to give, at 51% conversion, tert-butyl (1R,2S,3R,alphaS)-3-methyl-2-N-benzyl-N-alpha-methylbenzylaminocyclopentane-1-carboxylate consistent with E > 130, and in 39% yield and 99 +/- 0.5% de after purification. Subsequent deprotection by hydrogenolysis and ester hydrolysis gives (1R,2S,3R)-3-methylcispentacin in > 98% de and 98 +/- 1% ee. Selective epimerisation of tert-butyl (1R,2S,3R,alphaS)-3-methyl-2-N-benzyl-N-alpha-methylbenzylaminocyclopentane-1-carboxylate by treatment with KO'Bu in 'BuOH gives tert-butyl (1S,2S,3R,alphaS)-3-methyl-2-N-benzyl-N-alpha-methylbenzylaminocyclopentane-1-carboxylate in quantitative yield and in > 98% de, with subsequent deprotection by hydrogenolysis and ester hydrolysis giving (1S,2S,3R)-3-methyltranspentacin hydrochloride in > 98% de and 97 +/- 1% ee.     

Enzymatic resolution of methyl (1RS)-N-tBoc-6-hydroxy-3,4-dihydro-1H-isoquinoline-1-carboxylate by Seaprose S

An efficient biocatalytic process has been developed for the resolution of methyl (1RS)-N-tBoc-6-hydroxy-3,4-dihydro-1H-isoquinoline-1-carboxylate rac-1 by means of Seaprose S-mediated enantioselective hydrolysis to afford (1S)-2 and (1R)-1 in 87% and 93% isolated yield, 101% and 96% potency, and ee >99.8% and >99.5%, respectively.     

Asymmetric synthesis of (1R,2S,3R)-gamma-methyl-cis-pentacin by a kinetic resolution protocol

The asymmetric synthesis of (1R,2S,3R)-3-methyl-2-amino-cyclopentane carboxylic acid has been achieved via kinetic resolution of racemic tert-butyl 3-methyl-cyclopentene-1-carboxylate with homochiral lithium (S)-N-benzyl-N-alpha-methylbenzylamide.     

First synthesis of (1S,2S)- and (1R,2R)-1-amino-2-isopropylcyclobutanecarboxylic acids by asymmetric Strecker reaction from 2-substituted cyclobutanones

An efficient and easy one-pot reaction from readily available racemic 2-substituted cyclobutanones gave, by means of asymmetric Strecker synthesis in the presence of an amine chiral auxiliary, two major aminonitriles with excellent diastereoselectivity. After separation, the major cis-aminonitriles were hydrolysed and hydrogenolysed to lead for the first time to pure non-racemic (+)-1-amino-2-isopropylcyclobutanecarboxylic acid (ACBC) and its antipode.     

New route to (3RS) (5R) (5-2H)- and (3RS) (5S) (5-2H)-mevalonolactones

The synthesis of mevalonolactone stereospecifically labeled at carbon 5 is described combining chemical reactions with an enzymatic reduction step.     

(5S,6R)-2-(4-甲基)苯基-6-[(1R)-叔丁基二甲基硅氧乙基]-青霉烯-3-羧酸乙酯的合成研究

青霉烯[1]类抗生素是近年来发展很快的一种非典型β-内酰胺抗生素.青霉烯类化合物[2]具有广泛的抗菌活性,具有优于碳青霉烯类抗生素的特点,其固体化合物和酯型前药可口服吸收,不易被β-内酰胺酶水解,同时对脱氢肽水解酶-I( DHP-I )较碳青霉烯稳定.     

Uncommon transformations of methyl (1S,2S,3R,4R)-2,3-isopropylidenedioxy-5-iodomethyl-2-tetrahydrofurylacetate initiated by bases

Methyl (1S,2S,3R,4R)-2,3-isopropylidenedioxy-5-iodomethyl-2-tetrahydrofurylacetate prepared in two stages from D-ribose acetonide underwent a series of uncommon transformations under the treatment with bases providing the following different products depending on the base applied: methyl 3-(5-acetyl-2,2-dimethyl-1,3-dioxol-4-yl)propionate (DBU), methyl 2,3-isopropylidenedioxy-7-oxabicyclo[2.2.1]heptane-6-carboxylate (t-BuOK), methyl {(5R)-2,2-dimethyl-5-[(2R)-oxiranyl]-1,3-dioxolan-4-ylidene}propionate and methyl-(E)-3-{(4S,5R)-2,2-dimethyl-5-[(1R)-(2-oxiranyl)]-1,3-dioxolan-4-yl}-2-propenoate (t-BuOK and LDA).     

Parallel kinetic resolution of tert-butyl (RS)-3-alkyl-cyclopentene-1-carboxylates for the asymmetric synthesis of 3-alkyl-cispentacin derivatives

The double mutual kinetic resolution of tert-butyl (RS)-3-benzyl-cyclopentene-1-carboxylate with a 50 : 50 mixture of lithium (RS)-N-benzyl-N-alpha-methylbenzylamide and lithium (RS)-N-3,4-dimethoxybenzyl-N-alpha-methylbenzylamide gives, after protonation with 2,6-di-tert-butylphenol, a 50 : 50 mixture of the readily separable N-benzyl-(1SR,2RS,3RS,alphaRS)- and N-3,4-dimethoxybenzyl-(1SR,2RS,3RS,alphaRS)-beta-amino esters in >98% de in each case. This product distribution indicates that these amides react at very similar rates and with no mutual interference to furnish readily separable products, and are thus ideal for parallel kinetic resolution. The efficient parallel kinetic resolution (E > 65) of a range of tert-butyl (RS)-3-alkyl-cyclopentene-1-carboxylates with a pseudoenantiomeric mixture of homochiral lithium (S)-N-benzyl-N-alpha-methylbenzylamide and lithium (R)-N-3,4-dimethoxybenzyl-N-alpha-methylbenzylamide gives, after separation and N-deprotection, a range of carboxylate protected 3-alkyl-cispentacin derivatives in >98% de and >95% ee.     

Synthesis of potential metabolites of the brain imaging agents methyl (1R,2S,3S,5S)-3-(4-Iodophenyl)-8-alkyl-8-azabicyclo[3.2.1]octane-2-carboxylate

The synthesis of potential hydroxy metabolites of the brain imaging agents methyl (1R,2S,3S,5S)-3-(4-iodophenyl)-8-methyl-8-azabicyclo[3.2.1]octane-2-carboxylate and methyl (1R,2S,3S,5S)-3-(4-iodophenyl)-8-(3-fluoropropyl)-8-azabicyclo[3.2.1]octane-2-carboxylate are reported. The nitration of iodophenyltropanes 1 or 2 with nitronium tetrafluoroborate afforded the nitro compounds 3 or 4 which were reduced with iron powder to the corresponding amino compounds 5 and 6 . The final hydroxylated products 7 and 8 were obtained via a modified Sandmeyer reaction.     

Preparation of Optically Pure (1R, 2S)- and (1S, 2R)-DI-p-Methoxyphenyl Amino Alcohol

Racemic di-p-methoxyphenyl amino alcohol was synthesized in three steps. Optically pure di-p-methoxyphenyl amino alcohols were obtained by recrystallization. The absolute configuration of the amino alcohol was determined by X-ray crystallography.     

Efficient enantioselective synthesis of (2R,3R)- and (2S,3S)-3-hydroxyleucines and their diastereomers through dynamic kinetic resolution

(2R,3R)- and (2S,3S)-3-Hydroxyleucines, components of cyclodepsipeptides, papuamides and polyoxypeptins, were efficiently synthesized along with their diastereomers from the corresponding β-keto-α-amino acid ester through dynamic kinetic resolution using RuCl₂(binap)-catalyzed hydrogenation.     

Synthesis of Ethyl (2RS,3SR)-1-Tosyl-3-Vinylazetidine-2-carboxylate and ethyl (2RS,E)-3-ethylidene-1-tosylazetidine-2-carboxylate (=rac-ethyl N-tosylpolyximate)

The synthesis of 3-ethylideneazetidine-2-carboxylic acid (=polyoximic acid; 3 ) is a approached in two different ways leading to potential precursors of 3 . The first way involved a ring closure to a vinyl-subsatituted azetidin. Thus, Ireland-Claisen rearrangement of the Boc-glycinates 6 and 10 of (Z)- and (E)-2-butene-1,4-diol afforded, after exchange of the N-protecting groups, the isomeric 2-(tosylamino)-3-vinylbutanolides 13 and 14 with high stereoselectivity. Only the cis-isomer 14 could be further transformed to 3-(bromomethyl)-2-(tosylamino)-4-pentenoate 17 , and in a smoth cyclization with K₂CO₃, to trans-3-vinylazetidtene-2-carboxylaze 18 (Scheme 2). In the second approach, the 3-ethylidene isomer 19 of 18 was obtained more directly by a [2+2] cycloaddition, together with the two isomers 23 and 24 , from methlallene 20 and (tosyliminno)acetate 21 (Scheme 3). The main product of this reaction was, however, 2-(tosylamino)-4-hexinoate 22 , the product of an ene reaction.