Hydrides oxide, stereochemistry

An asymmetric variant of this reaction was developed using chiral Pd complex 111 with either silanes or disiloxanes [66-68]. Both relative and absolute stereochemistries were controlled in this system and good yields (60-85%) were obtained after oxidation (Eq. 18). Formation of the silane-containing product was inhibited by the presence of water due to competitive formation of the palladium hydrides and silanols [68]. The use of disiloxanes as reductants, however, provided expedient oxidation to the alcohol products without decreasing the isolated yields enantioselectivity was 5-15% lower in this more robust system [66]. Benzhydryldimethylsilane proved to be a good compromise between high yield and facile oxidation [66]. Palladium com-... 

Mori has reported the nickel-catalyzed cyclization/hydrosilylation of dienals to form protected alkenylcycloalk-anols." For example, reaction of 4-benzyloxymethyl-5,7-octadienal 48a and triethylsilane catalyzed by a 1 2 mixture of Ni(GOD)2 and PPhs in toluene at room temperature gave the silyloxycyclopentane 49a in 70% yield with exclusive formation of the m,//7 //i -diastereomer (Scheme 14). In a similar manner, the 6,8-nonadienal 48b underwent nickel-catalyzed reaction to form silyloxycyclohexane 49b in 71% yield with exclusive formation of the // /i ,// /i -diastereomer, and the 7,9-decadienal 48c underwent reaction to form silyloxycycloheptane 49c in 66% yield with undetermined stereochemistry (Scheme 14). On the basis of related stoichiometric experiments, Mori proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of dienals involving initial insertion of the diene moiety into the Ni-H bond of a silylnickel hydride complex to form the (7r-allyl)nickel silyl complex li (Scheme 15). Intramolecular carbometallation followed by O-Si reductive elimination and H-Si oxidative addition would release the silyloxycycloalkane with regeneration of the active silylnickel hydride catalyst. 

The Homer-Emmons addition of dialkyl carboalkoxymethylenephosphonates to aldehydes [22] has been widely used to generate a,p-unsaturated esters which, in turn, can be reduced to allylic alcohols. Under the original conditions of the Homer-Emmons reaction, the stereochemistry of the oc,(3-unsaturated ester is predominantly trans and therefore the trans allylic alcohol is obtained upon reduction. Still and Gennari have introduced an important modification of the Homer-Emmons reaction, which shifts the stereochemistry of the a,[i-unsaturated ester to predominantly cis [23], Diisobutylaluminum hydride (DIBAL) has frequently been used for reduction of the alkoxycarbonyl to the primary alcohol functionality. The aldehyde needed for reaction with the Homer-Emmons reagent may be derived via Swern oxidation [24] of a primary alcohol. The net result is that one frequently sees the reaction sequence shown in Eq. 6A. 1 used for the net preparation of 3E and 3Z allylic alcohols. 

The transformation of tazettine (9) into pretazettine (11 R = OH) (Scheme 1) confirms the stereochemistry of the latter alkaloid. Reduction of tazettine with lithium aluminium hydride gives a mixture of diols that are epimeric at C-3, cf. (10), presumably via a keto-alcohol. Compound (10) was made to cyclize to desoxypretazettine (11 R = H), which was shown by o.r.d., c.d., and H n.m.r. data to have the same configuration at C-6a as pretazettine (11 R = OH). Oxidation of compound (10) gave pretazettine, without affecting the configuration at C-3.7... 

The temporary silicon connection method introduced by Stork is a useful protocol for the preparation of silaoxacycles. The C-Si bond in the silaoxacycles is readily convened to C-OH with retention of stereochemistry by oxidative procedures. Utimoto and co-workers have developed a novel method for the synthesis of triols using the temporary silicon connection method [95BCSJ625], Treatment of the silacycle 184 with tin hydride furnished a mixture of silacycles, which were converted to the triacetate 187 without isolation. The endo isomer 185 was formed as the major compound. 

Thiazolylium and benzothiazolylium salts react normally with aqueous sodium borohy-dride yielding the corresponding thiazolidine or benzothiazoline. The mechanism and the stereochemistry of the reaction have been studied with thiazolylium salts chosen as models for thiamine, using borodeuteride/hydride and deuterium/protium oxide. The pathway described in Scheme 30 was suggested it involves the addition of a nucleophilic hydride at C-2 (50 -> 51), the addition of an electrophilic proton at C-5 (51 -> 52) and the addition of a second nucleophilic hydride at C-4 (52 — 53). 

The stereochemistry of the reaction in which C-4 is oxidized to give intermediate 41 is difficult to envisage, because the hydride abstraction and addition would have to take place from opposite sides of the carbonyl group. Various solutions to this problem have included (a) a double binding-site for the substrate, which can transfer from one site to the other as the intermediate 4-ulose,146... 

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