1.3- Oxathiane stereoselectivity

Oxathianes Dehydration of 2,2 -bis(hydroxyethyl) sulfides with (C6H5),P(OC2H5)2 provides a stereoselective route to 1,4-oxathianes in moderate (isolated) yields. 

Also, the reaction pathways of the Corey-Chaykovsky epoxidation reaction have been compared quantum-chemically <1999JOC4596>. As models for one transition state, 1,3-oxathiane compounds such as 52, suitably substituted to allow comparison with experiment (Equation 3), were calculated and these predicted both the absolute stereochemistry of the main product 53 and the distribution of the other stereoisomers, as supported by experimental results. Thus, this theoretical study was able to identify the transition state which proved to be responsible for the stereoselectivity of the catalytic Corey-Chaykovsky epoxidation reaction. 

Eliel s oxathiane auxiliary was used for stereoselective transformations and has been reviewed in part <2003H(60)1477>. As expected, reaction of the lithiated auxiliary with acetaldehyde gave the addition product with low stereoselectivity at the side-chain stereocenter <1997M201>. Better stereocontrol was observed, when methyl Grignard reagent was added to 2-acyl-l,3-oxathiane <2000JCCS63>. Reaction of 2-vinyl-l,3-oxathiane with 1,1-diphenylethene, mediated by TiCU, afforded dihydrothiopyrans in 82% yield, albeit with low enantioselectivity (Scheme 84) <2003T1859>. 

Disubstituted 1,3-oxathianes 316, prepared from benzaldehyde and the corresponding 3-mercaptoalkanol, have been stereoselectively deprotonated at the equatorial position to yield, after reaction with electrophiles, 2,2-disubstituted products. t.S j-Benzoin was obtained in 75% ee in the case of using compound 316 (R = OTBS), after reaction with benzaldehyde and final deprotection of the major product 317 with NCS and silver nitrate (Scheme 83)490. 

Efficient syntheses of tetrahydrothiopyran-4-one and 3,6-dihydro-4-trimethylsilyloxy-277-thiopyran 58 from dimethyl 3,3 -thiobispropanoate have been described <07S1584>. Carbene insertion into a C-H bond of diazosulfones leads to tetrahydrothiopyran sulfones 59 with stereoselectivity for the trans disposition of the 2- and 3-substituents. When applied to diazosulfonates, the corresponding 1,2-oxathianes are produced <07OL61>. 

In theory, the epimeric a-hydroxy aldehydes can be obtained by reversing the order of introduction of R and R. When this procedure is impracticable or results in lower stereoselectivity, the oxathiane 2 can be used in plate of 1. 

Stereoselective synthesis of oxathiane tertiary carbinols (8, 508-509). This reagent is now preferred over the isomeric 4,6,6-trimcthyl-l,3-oxathiane (8, 508-509) for a two-step, highly stereoselective synthesis of oxathiane tertiary carbinols (3, equation I), which can be cleaved to enantiomerically pure tertiary a-hydroxy aldehydes. The first step, acylation with retention of configuration, gives the more stable equatorial adduct 2... 

Methylthiohexanol and cis/trans-2-methy -4-pvopy -1,3-oxathiane have been reported by Winter et al. [71] to play important roles in the delicate flavour of the yellow passionfruit, in spite of their occurrence in the extreme trace levels. Using complexation GC, the stereoselective analysis of all the four stereoisomers of 2-methyl-4-propyl-l,3-oxathiane was achieved. This was the first direct stereochemical... 

Chiral 1,3-oxathianes have been used as adjuvcuits for highly stereoselective asymmetric syntheses. 

Addition of Chiral Ligands. If a chiral adjuvant is used to achieve asymmetric induction, it should preferentially be inexpensive, easily removed, efficiently recovered, and capable of inducing high stereoselectivity. Of these, 1,3-oxathianes based on (-l-)-camphor or (-l-)-pulegone, and proline-derived 1,3-diamines, have been accorded the greatest attention. 

Arylalkylketenes undergo stereoselective [2 + 2 - - 2] cycloaddition with carbon disulfide at — 40 °C using toluene as the solvent, giving only the E isomer of l,3-oxathian-6-ones 525 in a reaction catalyzed by the NHC from 46 (Scheme 174) (2011JCS(CC)8388). The calculated results indicate that this reaction takes place in four steps the process starts with the complexation of NHC with ketene rather than with CS2, followed by addition of CS2, subsequent conventional [4 - - 2] cycloaddition with the second molecule of ketene and at last regeneration of catalyst. The second step was found to be the rate-determining step (2013JOC11849). 

Ethoxycarbonylnitrene (generated photochemically from the azide) also reacts with 4-t-butylthian, affording a nearly 1 1 mixture of both cis- and trans-sulphimides. This non-stereoselectivity in the reaction of the nitrene is in marked contrast with that of the reaction of carbenes with the thian the thian reacts with both bis(alkoxycarbonyl)- and bis(diacetyl)-carbenes, giving the trans-yMes as the sole products (see Vol. 4, p. 112). Various sulphimides of cyclic sulphides, e.g. thians, cis- and frans-thiadecalins, oxathians, and dithians, can be prepared by treating the sulphides either with anilines and AT-chlorosuccinimide or with Bu OCl. In this reaction, the formation of (33 R = Bu , = /7-ClCjH4) with an equatorial... 

The j -BuLi-(—)-sparteine combination is widely used in enan-tioinductive organic reactions. 4.H7-i22 Benzylic asymmetric alkylation may thus be achieved by subsequent treatment with alkyl halides, alkyl tosylates, and allyl tosylates 1,3-Oxathiane derivatives shown in eq 50 may be lithiated and quenched with electrophiles. An interesting aspect of this reaction is the equatorial stereoselectivity, which is independent of the size of the electrophile introduced. ... 

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