Trisubstituted oxiranes

The enantioconvergent biohydrolysis of sterically demanding trisubstituted oxiranes has also been reported [189,190]. For instance, the enantioconvergent hydrolysis of a trisubstituted rac-epoxide (Figure 6.70) was a key step in the chemoenzymatic synthesis of a volatile constituent of the beer aroma [190]. 

Nucleophilic reagents will also effect deoxygenation. Di- or trisubstituted oxiranes have been transformed to olefins stereospecifically with inversion via a phosphobetaine (Eq. 102). ... 

The unusual reactivity of selenoboranes towards epoxides gives new selective routes to /3-hydroxy-selenides and allyl alcohols.Thus, 1,2-epoxyoctane with B(SeMe)3 at 0°C for 0.7 h, followed by aqueous NaHCOa, gave (164) (81%) whereas the same reaction with styrene oxide gave (165) (63%) as the major product. For trisubstituted oxirans, however, the products were allylic alcohols, e.g. (167) (76%) from (166) on treatment with B(SePh)3 at 20 °C for 1.5 h. 

In contrast to their reaction with R R CuLi (R = alkyl, R = alkyl or CN), the reactions of epoxides with the higher order mixed organocuprates R2Cu(CN)Li2 (R = various alkyl or aryl) give excellent yields of ring-opened products. The trisubstituted oxiran (190), on reaction with Pr"Cu(CN)Li, affords only 23% of (191), but with Pr"2Cu(CN)Li2 the yield is increased to 86%. 

Of course, epoxides can also undergo many modes of chemistry in which the heterocyclic ring remains intact. One such example is the addition of lithiated oxazolinyl epoxides (114) onto carbonyl compounds, providing a synthesis of trisubstituted oxiranes (116). In this case the oxazolinyl group not only confers stability to the lithiate, but also provides an opportunity to introduce chiral auxiliaries <01JOC3049>. 

Steinreiber, A., Mayer, S.R, Saf, R. and Faber, K. (2001) Biocatalytic asymmetric enantioconvergent hydrolysis of trisubstituted oxiranes. Tetrahedron Asymmetry, 12,1519-1528. 

Another strategy toward an enantioconvergent process was achieved by combination of bio- and chemocatalysis (see also 2,2-di- and trisubstituted oxiranes). Thus, in a first... 

For steroids bearing two trisubstituted double bonds, such as A - and 11 -dienes, e.g. (35), the 5,6-oxirane is formed preferentially. The 9(ll)-double bond in -dienes, e.g. (38), is usually attacked first by... 

Head-to-head [2+2]photocycloaddition of 1,2-diarylethanediones and 2-aminopropene nitriles (CH2 C(CN)NR2) occurs to yield oxetanes 10 in moderate to good yields. The formation of only one diastereoisomer in each of the cases investigated is rationalized in terms of the most easily accessible and stabilized 1,4-diradical intermediate <95RTC498>. 2,3,4-Trisubstituted oxetanes 11 are obtained in high yield by intramolecular nucleophilic attack of the anion from certain 2-(l-alkoxyethyl)-3-substituted oxiranes <96JOC4466>. 

Scheme 6.20. This ruthenium catalyst (10 mol%) was active for the cydization of ds-1 -ethynyl-2-vinyloxiranes to afford various 2,6-disubstituted phenols in reasonable yields. Under similar conditions, 1,1,2,2,-tetrasubstituted oxiranes gave the 2,3,6-trisubstituted phenols with a skeleton reorganization [22]. The 1,2-deuterium shift of the alkynyl deuterium of d-Sle was indicative of mthenium vinylidene intermediates (Scheme 6.20).

The keto group of the trisubstituted dioxanone 133 generated by ozonolysis was removed by radical deoxygenation according to the Barton-McCombie protocol [80] via the alcohols 134 and the corresponding xanthate, leading after deben-zylation to the dioxane 135 in excellent yield. After conversion to the tosylate, cleavage of the acetonide and protection of the secondary alcohol function as a TBS ether provided access to oxirane 128 by cyclization with NaH in 99% yield and in virtually diastereo- and enantiomerically pure form (de, ee > 96%). 

Interestingly, when aryl oxirane ethers derived from secondary alcohols were reacted under the same conditions, the corresponding oxetanes were formed exclusively <1996JOC4466>. This was again a highly stereoselective reaction, forming trisubstituted oxetanes with four stereocenters. 

Trisubstituted okfins. The oxirane (1) is converted into (2a), (E)-5-bromo-2-methyl-2-pcntenc-l-ol, in 81 % yield (pure) when treated with 48% hydrobromic acid at 0 for 1 hr. Alternatively, (I) is transformed into (2b) by treatment with sodium... 

Oxiranes are transformed by tris(ethylthio)borane to sulfur-containing derivatives.With selenoboranes, terminal or a,)3-disubstituted epoxides yield 3-hydroxyse enides trisubstituted epoxides give allyl alcohols. 

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