Stereoselective pinacol-type

A speculative biosynthetic pathway for trehazolin (2) has been proposed,42 as shown in Scheme 1. Two molecules of the glucosylamine (i) could react with carbon dioxide to give the carbodiimide ii, which could be transformed into iii. Subsequent regioselective oxidation to iv, followed by a stereoselective pinacol-type coupling could afford trehazolin (2). The biosynthesis of trehalose has also been the subject of several subsequent chapters.43-48... 

Similar stereoselectivities are achieved in the allylation of enantiomerically pure proline-derived a-oxoamides47. l-Bromo-3-methy]-2-butcne reacts with clean allylic inversion. Since pinacol-type coupling products are also produced under the reaction conditions, this was taken as evidence for a radical addition mechanism47. 

Intramolecular reaction of an allylsilane and an aldehyde was performed to prepare cyclic compounds as illustrated in Eq. (90) [233], which shows a high diastereoselec-tivity. The cyclization of optically active allylsilane proceeds stereoselectively in a manner consistent with the antiS l mechanism (Eq. 91) [234]. Development of a new reagent with a bis-allylsilane moiety effected tandem inter- and intramolecular cyclizations to give cyclic compounds as exemplified in Eqs (92) [235] and (93) [236-238]. In Eq. (92), the double addition product initially formed underwent a pinacol-type rearrangement under the influence of TiCU to give, eventually, the methyl cyclopentyl ketone. Further examples of intramolecular cyclization of allylsilanes are summarized in Table 8. 

Epoxide opening accompanied by the pinacol-type rearrangement of a carbon framework in the presence of a titanium Lewis acid has been used for the stereoselective synthesis of acyclic compounds [642,643], Equations (278) and (279) illustrate this transformation and exemplify the dependence of stereospecificity on the geometry of the epoxide... 

When a,j9-unsaturated aldehydes were exposed to a stoichiometric or a catalytic amount of Cp2TiCl2 and s-BuMgCl, pinacol-type coupling products were isolated in good yields [Eq. (179) 423,424]. The excellent stereoselectivity found in the products is noteworthy. 

The coupling of carbonyl compounds " to give 1,2-diols, known as the pinacol coupling, has been carried out in aqueous media. Clerici and Porta extensively studied the aqueous pinacol coupling reactions mediated by Ti(III). Schwartz reported a stereoselective pinacol coupling with a cyclopentadienetitanium complex. Pinacol-type couplings were also developed by using a Zn-Cu couple, Mn, °... 

Pinacol-type rearrangement of cis-diol 26.137 by esterification with TsCl in pyridine at rt readily proceeded with high stereoselectivity to afford directly the azulenofuran 26.138 (Scheme 40). However, direct... 

High stereoselectivity for the d,l product diastereomer has been found in the photochemically induced pinacol-type coupling of aldehydes by tristannanes (Scheme 35). The stannadioxolans (55) produced may be worked-up with acetyl chloride (or trimethylsilyl chloride) to give derivatives of 1,2-diols. 

Self-condensation of the substituted propiophenone, 15, by the pinacol reaction proceeds to give the glycol, 16, as the meso isomer. (If it is assumed that the transition state for this reaction resembles product, this stereoselectivity can be rationalized on the grounds of steric interaction compare A, which leads to the observed product, with B.) Dehydration under very specialized conditions (acetyl chloride, acetic anhydride) affords the bisstyrene-type diene (17). Removal of the acyl groups by means of base affords the synthetic estrogen, dien-... 

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