Synthesis of alexine-like compounds from chiral five-membered endocyclic enecarbamates

Stereoselective syntheses of enantiomerically enriched trihydroxy pyrrolizidine and indolizidines were accomplished from a common chiral endocyclic enecarbamate. The synthetic strategy features an efficient [2+2] cycloaddition of ketenes to the endocyclic enecarbamate and a highly regioselective Baeyer-Villiger oxidation of the intermediate azabicyclic-cyclobutanones. These new heterocycles are compounds structurally related to the alexines.     

Synthesis of dihydroxylated prolines and iminocyclitols from five-membered endocyclic enecarbamates. Total synthesis of the potent glycosidase inhibitor (2R,3R,4R,5R)-2,5-dihydroxymethyl-3,4-dihydroxypyrrolidine (DMDP)

cis- and trans-3,4-Dihydroxylated prolines and the iminocyclitol 1,4-dideoxy-1,4-imino ribitol were synthesized employing a strategy involving the Heck arylation of five-membered endocyclic enecarbamates with aryldiazonium salts followed by oxidative cleavage of the electron-rich aromatic ring. The total synthesis of the potent α- and β-glucosidase inhibitor (2R,3R,4R,5R)-2,5-hydroxymethyl-3,4-dihydroxypyrrolidine (DMDP) was also achieved by the same strategy in ten steps from a chiral five-membered enecarbamate in 12% overall yield.     

Synthesis of the 3-(3,4,5-Trimethoxyphenyl)-pyrrolidine: A New Conformationally Constrained Mescaline Analogue

The total synthesis of the 3‐(3,4,5-trimethoxyphenyl)-pyrrolidine, a new and conformationally constrained mescaline analogue, was accomplished in a concise and efficient manner. The synthetic route encompassed only 4 steps from the starting N-Cbz-3-pyrrolidine, in 46% overall yield. The route features a highly effective Heck arylation of the non-activated olefin N-Cbz-3-pyrrolidine with the 3,4,5-trimethoxybenzene diazonium tetrafluoroborate. The hemiaminal (lactamol) Heck adduct was converted to the mescaline analogue in a sequence of reactions: (a) dehydration of the intermediate hemiaminal 3 with trifluoroacetic acid anhydride, (b) hydrogenation/hydrogenolysis of the endocyclic enecarbamate 6 with H2-Pd/C, and (c) formation of the rather stable mescaline analogue in the form of a hydrochloride salt. The target molecule constitutes a new mescaline analogue with potential activity towards 5‐HT2 dopamine receptors.     

A Concise Stereoselective Synthesis of (R)‐2‐Benzylmorpholine and ML398 from (R)‐(−)‐2‐Phenylglycinol

We describe here an efficient stereoselective method for the preparation of (R)‐2‐benzylmorpholine and ML398. The present method features a high diastereocontrol using an endocyclic oxidation of phenylglycinol‐derived morpholine and a stereoselective alkylation of chiral non‐racemic morpholin‐3‐one as key steps.     

Stereoselective reduction of endocyclic carbon–nitrogen double bond: application to the synthesis of biomolecules

Recent advances in the stereoselective synthesis of nitrogen containing heterocyclic compounds using the reduction of endocyclic double carbon–nitrogen bond are surveyed.     

Total synthesis of (-)-nakadomarin A

A concise diastereoselective total synthesis of (-)-nakadomarin A has been completed in 21 steps from D-pyroglutamic acid. Key steps include an enecarbamate Michael addition/furan-N-acyliminium ion cascade cyclization to provide the tetracyclic core and ring-closing alkyne and alkene metatheses to construct the fifteen- and eight-membered azacycles, respectively.     

Cycloaddition of P−C Single Bonds: Stereoselective Formation of Benzo‐1,3,6,2‐trioxaphosphepine Complexes via a Ditopic van der Waals Complex

While phosphaalkenes and phosphanes are known to participate in [4+n] cycloaddition reactions, P?C single bonds are inert in this respect. Herein, reactions of oxaphosphirane complexes with tetrachloro‐ortho‐benzoquinone are presented that reveal a stereoselective reaction of the endocyclic P?C bond to afford benzo‐1,3,6,2‐trioxaphosphepine complexes. High‐level DFT calculations provide evidence that the final product is derived from a sequence of three consecutive steps involving a ditopic van der Waals complex.     

Synthesis of indolizidines and pyrrolizidines through the [2 + 2]cycloaddition of five-membered endocyclic enecarbamates to alkyl ketenes. Unusual regioselectivity of Baeyer-Villiger ring expansions of alkyl aza-bicyclic cyclobutanones

The total syntheses of two indolizidine skeletons and of the necine base (+/-)-platynecine were accomplished in a concise manner with good overall yields starting from a common five-membered endocyclic enecarbamate. These syntheses feature a [2 + 2]cycloaddition of the five-membered endocyclic enecarbamate 5 to alkylketenes that proceeded in high yields and with high stereoselectivity to provide an endo alkyl cycloadduct as the major or only product. The minor exo alkyl cycloadducts, which can be observed in some [2 + 2]cycloadditions, seem to derive from the endo cycloadduct, the putative kinetic product, by epimerization. An unusual regioselectivity was observed for the Baeyer-Villiger oxidation of 7-alkyl-2-azabicyclic cyclobutanones. Endo-7-alkyl cycloadducts ring-expanded exclusively to a gamma-lactone in which oxygen is inserted into the C6-C7 bond in preference to the bridgehead C5-C6 bond. With the exo-7-alkyl cycloadduct the regioselectivity of the Baeyer-Villiger oxidation is drastically reduced, leading to mixtures of regioisomeric lactones in a ratio of approximately 1.5 to 1. It is hypothesized that the steric strain built into the Criegee cyclobutane intermediate is the regioselective controlling factor in these oxidations, overriding any stereoelectronic bias for ring expansion. A rationale for the mechanism of the [2 + 2]cycloaddition involving enecarbamates and ketenes is presented, which seems to involve the participation of an N-acyliminium-enolate intermediate.     

Total synthesis of 8-methoxygoniodiol related compounds via chiron approach

The stereoselective synthesis of 8-methoxygoniodiol related compounds was accomplished by using readily available δ-gluconolactone as a chiral source. The stereoselective addition of an aryl Grignard reagent on an aldehyde, stereoselective reduction of a keto group and regioselective opening of a chiral epoxide by ethyl propiolate are the key steps involved in this synthesis.     

Stereoselective synthesis of an advanced seco ester intermediate as a precursor toward the synthesis of amphidinolides T1, T3, and T4

An efficient, convergent, and stereoselective synthesis of a very advanced intermediate toward the total synthesis of amphidinolides T1, T3, and T4 utilising Evan’s aldol and alkylation reactions, oxy-Michael, cross metathesis, stereoselective Grignard addition, and Yamaguchi esterification reactions as key steps is described.     

An efficient total synthesis of (-)-huperzine a

The total synthesis of Lycopodium alkaloid (-)-huperzine A has been accomplished in 10 steps with 17% overall yield from commercially abundant (R)-pulegone. The synthetic route features an efficient synthesis of 4 via a Buchwald-Hartwig coupling reaction, a dianion-mediated highly stereoselective alkylation of 4, and a rare example of an intramolecular Heck reaction of an enamine-type substrate. The stereoselective β-elimination and the accompanying Wagner-Meerwein rearrangement are of particular interest.     

Convergent, stereoselective synthesis of the GHIJ fragment of brevetoxin A

[reaction: see text] A stereoselective synthesis of the GHIJ fragment of brevetoxin A utilizing a convergent assembly strategy is described. Glycolate alkylation, ring-closing metathesis, and Hosomi-Sakurai reactions were central operations in the construction of the G ring and J ring subunits, which were united through a Horner-Wadsworth-Emmons coupling. Subsequent dehydrative cyclization produced an endocyclic enol ether that was further elaborated to the tetracyclic GHIJ fragment of brevetoxin A.     

A convergent coupling strategy for the formation of polycyclic ethers: stereoselective synthesis of the BCDE fragment of brevetoxin A

A stereoselective synthesis of the BCDE fragment of brevetoxin A has been completed. anti-Glycolate aldol, glycolate alkylation, and ring-closing metathesis reactions were employed as key bond-forming events. A convergent assembly strategy was employed that relied on a Horner-Wadsworth-Emmons union of two complex fragments. Subsequent cyclization and dehydration led to efficient generation of an intermediate endocyclic enol ether, which was advanced to a tetracyclic fragment. [reaction: see text]     

Fischer carbene catalysis of alkynol cycloisomerization: application to the synthesis of the altromycin B disaccharide

[reaction: see text] The tungsten-catalyzed cycloisomerization of alkynyl alcohols can be conducted without using photochemistry, using a stable tungsten Fischer carbene as the precatalyst for this transformation. A variety of alkynyl alcohols undergo cycloisomerization under these conditions to provide endocyclic enol ethers of five-, six-, and seven-membered ring sizes. The utility of this method is further demonstrated in the stereoselective synthesis of the disaccharide substructure of altromycin B.     

Synthesis of the apoptosis inducing agent apoptolidin. Assembly of the C(16)-C(28) fragment

[reaction--see text] A stereoselective synthesis of the C(16)-C(28) fragment of the apoptosis inducing agent apoptolidin is described. Key steps include two propionate aldol reactions and a stereoselective Mukaiyama aldol addition of enolsilane 19 to beta-methoxy aldehyde 4.     

A short total synthesis of (±)-epimeloscine and (±)-meloscine enabled by a cascade radical annulation of a divinylcyclopropane

The first stereoselective synthesis of epimeloscine has been accomplished in 13 total steps with a longest linear sequence of 10 steps. The core of the synthesis takes only five steps, the key ones being acylation, stereoselective tandem radical cyclization of a divinylcyclopropane to make two rings, and group-selective ring-closing metathesis of the resulting divinylcyclopentane to make the last ring.     

Total synthesis of (-)-salinosporamide A

A concise and stereoselective total synthesis of (-)-salinosporamide A (1), a potent inhibitor of the 20S proteasome that is in clinical development as an anticancer drug candidate, has been accomplished in 14 steps with 19% overall yield from 4-pentenoic acid. Our synthesis features a stereoselective alkylation utilizing a chiral auxiliary, formation of a pyrrolidine unit, and oxidation of the pyrrolidine to a γ-lactam. To demonstrate the scalability of our synthesis, (-)-salinosporamide A has been synthesized on a gram scale.     

Regioselective epoxide ring opening. Steroselective synthesis of a tetrahydropyran ring

The stereoselective synthesis of a 2-substituted tetrahydropyran with adjacent alkoxy-bearing stereogenic centre is described. The key steps of this synthesis were the stereoselective epoxidation of an allylic alcohol and the regioselective epoxide ring opening by lithium aluminum hydride. The regio and stereoselective synthesis of a trihydroxyselenide and a trihydroxysulfide is also described. The latter compounds are not suitable for cyclization to tetrahydrofuran ring.     

An enantioselective synthesis of the bicyclic core of the marine natural product awajanomycin

A concise stereoselective synthesis of the N-benzylated bicyclic core of the marine natural product awajanomycin is described starting with naturally occurring l-alanine. Key steps in the synthesis include a ring-closing metathesis to establish a δ-lactam ring followed by a stereoselective hydroxylation to instal the quaternary centre.     

Stereoselective synthesis of methyl 7-dihydro-trioxacarcinoside B

A stereoselective synthesis of 7-dihydro-triocacarcinose B, a branched octose from the quinocyclines, has been achieved. The biocatalytic resolution of a Baylis-Hillman adduct and a subsequent ring-closing metathesis were used to assemble the molecular framework. Subsequent key steps were a highly stereoselective epoxidation and a regio- and stereoselective opening of the epoxide by allyl alcohol and HClO4 to introduce the C(3)-OH group in protected form. The 7-dihydro-triocacarcinose B could be converted into the corresponding 1,7-anhydrosugar.