Functionalized esters as bis-electrophiles in a silicon-induced domino synthesis of annulated carbocycles

The reaction of silyl-substituted carbanion 1b with arene-1,2-dicarboxylates 6, 15 yields indenone derivatives 11, 16 in a domino process involving silyl C→O migration and elimination. However, in a competing pathway, the initial addition of 1b leads to lactone formation (8, 17). Substrates 26, 38 containing an ester group and a bromine substituent react with 1b under substitution of the halogen not allowing silyl migration. But desilylation with TBAF gives reactive carbanions providing benzo-annulated cycloalkanones 29, 40.     

Functional desymmetrization of 1,3-dioximes for the obtention of 1,2,3-hetero trisubstituted carbocycles

Carbocylic 1,3-dioximes react with acyl chlorides giving systems that may, upon heating, suffer [3,3]-sigmatropic rearrangements in high yields in only one of the oximes esters, yielding 1,3-dinitrogen-2-oxygen trisubstituted carbocycles. Use of more reactive electrophiles, such as p-toluenesulfonyl chloride and diethyl chlorophosphate, introduces the halogen at position 2, while cleaving the N-O bond of just one of the oxime functions.     

Versatile synthesis of cadalene and iso-cadalene from himachalene mixtures: Evidence and application of unprecedented rearrangements

From a mixture of α-, β- and γ-himachalenes extracted from waste wood of Atlas cedar (Cedrus atlantica), cadalene (1,6-dimethyl-4-isopropylnaphthalene) and iso-cadalene (1,6-dimethyl-3-isopropylnaphthalene) were produced in two steps with up to 71% ± 5% yield through the ar-himachalene intermediate using I₂ and/or AlCl₃ as reagents. The selectivity is shown to sharply depend on the operating conditions: while I₂/AlCl₃ in dichloromethane promotes the formation of cadalene, the formation of iso-cadalene is favored in the presence of AlCl₃ in cyclohexane. The bicyclic aromatic compounds were thus obtained through unique rearrangements involving sequential CC bond cleavage/formation and hydride transfer processes. In the absence of AlCl₃ or I₂, dihydrocurcumene was also found to be formed with up to 70% selectivity. A tentative mechanism is proposed and discussed.     

A cross-metathesis approach for the synthesis of tedanolide and 13-deoxytedanolide: stereoselective synthesis of the C3-C16 segment

A unified synthetic strategy has been devised for the synthesis of both tedanolide and 13-deoxytedanolide, which involves a cross-metathesis reaction as the key step. We report herein the stereoselective synthesis of the C3-C16 segment (+)-5b and subsequent manipulation of the C12-C13 double bond leading to the preparation of both tedanolides.     

Stereoselective synthesis of the C1-C10 fragment of rhizoxin D

A novel approach towards the synthesis of the C1-C10 fragment of the biologically active antimitotic agent rhizoxin D is described. The synthesis involves a stereoselective Michael addition reaction of lithium diallyl cuprate with an α,β-unsaturated six membered lactone.     

Stereoselective strategy for the synthesis of (+)-polyoxamic acid and some polyhydroxylated pyrrolidines

An efficient strategy for the synthesis of dihydroxy chiral amino moiety which can be utilized for the synthesis of polyoxamic acid, 1,4-dideoxy-1,4-imino-d-xylitol, and dihydroxy pyrrolidine by using highly diastereoselective nucleophilic addition on chiral imine has been reported.     

Studies toward the total synthesis of tedanolide: stereoselective synthesis of the C(8)-C(17) segment

The stereoselective synthesis of the C(8)-C(17) sub-unit (−)-5 of tedanolide (1), which involves iterative Evans aldol reactions as the key steps, is described.     

Intramolecular Heck reaction strategy for the synthesis of functionalised tetrahydroanthracenes: a facile formal total synthesis of the linear abietane diterpene, umbrosone

Rapid annulation employing an intramolecular Heck reaction yielded the functionalised 1,1,10-trimethyl-6-methoxy-1,2,3,4-tetrahydroanthracene 4a, a key intermediate for the linear diterpenoid quinone umbrosone (1), and the related compounds 4b-d. A similar strategy was also successfully adopted for the synthesis of the 9-methyl tetrahydroanthracene ester 5 and the tetrahydrodibenzo[a,d]cycloheptene ester 6.     

Protoilludane sesquiterpenes: synthesis of (±)-cerapicol, formal synthesis of (±)-sterpurene, and synthesis and absolute configuration of (+)-cerapicol

New approaches to the protoilludane sesquiterpenes (±)-cerapicol and (±)-sterpurene via rearrangement routes are described. The absolute configuration of (+)-cerapicol has been determined and found in accord with a biosynthesis of the natural product via cyclization of humulene to the so-called protoilludyl cation and a subsequent 1,2-alkyl shift.     

Percycloalkylated cyclohexanes: attempted synthesis of a trispropellane

The acid catalyzed rearrangement of two cyclohexanols of spiroannelated four-membered rings has been studied. In accordance with molecular mechanics calculations, far-reaching reorganizations with formation of unsaturated hexacyclic systems, including a fully cycloalkylated cyclohexene with a bispropellane partial structure, were observed. Attempts to convert this bispropellane to a trispropellane failed.     

A ring-closing metathesis based synthesis of bicyclic nucleosides locked in S-type conformations by hydroxyl functionalised 3′,4′-trans linkages

A [4.3.0]bicyclic nucleoside that contains an unsaturated hydroxylated 3′,4′-trans linkage has been efficiently synthesised. Thus, from diacetone-d-glucose as the starting material, stereoselective Grignard reactions for the introduction of allyl groups, a nucleobase coupling and, subsequently, a ring-closing metathesis (RCM)-reaction were applied as the key reactions. The cyclohexene moiety introduced in this nucleoside reveals a large potential for further derivatisation, and as the first example, a stereoselective dihydroxylation followed by deprotection afforded a multihydroxylated bicyclic nucleoside. The configuration and conformational behaviour was determined by NMR spectroscopy and ab initio calculations, and both this bicyclic nucleoside and its unsaturated analogue were found to be strongly restricted in S-type conformations.     

Surveying approaches to the formation of carbon-carbon bonds between a pyran and an adjacent ring

We have examined several methods for the stereoselective formation of carbon-carbon bonds between contiguous rings where a stereogenic center is already present. The approaches investigated were a [1,3] oxygen to carbon rearrangement of cyclic vinyl acetals, an intermolecular enolsilane addition into an in situ generated oxocarbenium ion, an intramolecular conjugate addition of tethered alkoxy enones, and epimerization of several α-pyranyl cycloalkanones. These routes have been found to be complementary in several cases and have enabled formation of both the trans:anti and cis:anti stereoisomers in good to excellent yields and varying diastereoselectivities. We have proven C2-C2′ relative stereochemistry of 1-2 via a chemical correlation.     

Isoselenocyanates: a powerful tool for the synthesis of selenium-containing heterocycles

Selenium-containing heterocyclic compounds have been well recognized, not only because of their remarkable reactivities and chemical properties, but also because of their diverse pharmaceutical applications. In this context, isoselenocyanates have been emerged as a powerful tool for the synthesis of selenium-containing heterocycles, since they are easy to prepare and store and are safe to handle. In this review the recent advances in the development of synthesis methods for selenium-containing heterocycles from isoselenocyanates are presented and discussed.     

Diastereoselective synthesis of the 5-hydroxy-pyrrolidinone amino acid of the microsclerodermins and model studies for an end-game strategy for microsclerodermin B

The first diastereoselective synthesis of the 5-hydroxy-pyrrolidinone amino acid common to eight members of the microsclerodermin family is presented. Our strategy involves formal hydration of an unsaturated precursor via the use of a two-step hydroxybromination-debromination protocol; this procedure provides exclusively the requisite 4,5-cis-pyrrolidinone. Furthermore model studies are presented that indicated the potential viability of this hydration strategy in the context of a synthesis of microsclerodermin B.     

Stereoselective synthesis of the C14-C24 degraded fragment of symbiodinolide

Symbiodinolide is a polyol macrolide isolated from the marine dinoflagellate Symbiodinium sp. in 2007. The C14-C24 fragment of symbiodinolide possessing the 17R/18R/21R absolute configuration, which was obtained as one of the degraded products of symbiodinolide, was synthesized stereoselectively from cis-2-butene-1,4-diol. The detailed comparison of the synthetic product with the degraded product in the spectroscopic data confirmed that the stereostructure of the C14-C24 fragment was 17R, 18R, and 21R.     

The diastereoselective synthesis of functionalised isoxazolidines using a Cope elimination/intramolecular nitrone cycloaddition strategy

Functionalised hydroxylamine derivatives of (S)-prolinol prepared by a Cope elimination have been found to undergo oxidation to the nitrone either in the presence of air or a catalytic quantity of TPAP. These undergo intramolecular cycloaddition to give tricyclic isoxazolidines with high diastereoselectivity.     

Stereoselective synthesis of the C1-C20 segment of the microsclerodermins A and B

An enantioselective route for the synthesis of key fragment C1-C20 resident in microsclerodermins A and B is described. The route features deoxygenative rearrangement of an hydroxy-alkynoate and a highly enantio- and diastereo-controled iterative dihydroxylation as key reactions, starting from S-(−)-citronellol.     

Synthetic studies toward the total synthesis of tedanolide: assembly of the C1-C23 carbon backbone

A stereoselective assembly of the C1-C23 fragment representing the carbon backbone of tedanolide was accomplished utilizing a chiral boron reagent to effect the aldol coupling of the C1-C12 diketoester fragment with the C13-C23 aldehyde fragment.     

Stereoselective synthesis of the C33-C44 fragment of palau’amide

An efficient stereoselective synthesis of the C33-C44 fragment of palau’amide is described using a Sharpless asymmetric epoxidation, a regioselective nucleophilic ring opening of the epoxide, a Grignard reaction and a Luche stereoselective reduction of a keto compound as the key steps.