Benzylidene derivatives hydrogenolysis

The 1-deazauridine (XU-587) has been prepared from 3-bromo-2,6-di-benzyloxypyridine by means of conversion to the 3-pyridyllithium and then to the di-3-pyridylcadmium (XU-584). Treatment of XII-584 with l-chloro-2,3,5-tri- -benzoyl-/3-D-ribofuranose gives a 2,6-dibenzyloxy-3-(D-ribofuranosyl)-pyridine (MI-585) and the benzylidene derivative (MI-586). Hydrogenolysis of XII-585 gives a 3-ribosyl-2,6-dihydroxypyridine (XII-587). 

Dicarboxylic acids, such as malonic acid, have been infrequently used in the form of protected cyclic isopropylidene or benzylidene derivatives, e.g. (15) and (22). The esters are saponified by base and as befits their acetal structures by acids also [56, 57, 67, 126]. Presumably the benzylidene ester may be cleaved by hydrogenolysis. Rather more use has been made of cyclic protecting groups in the case of a-hydroxyacids. Lemieux [68] thus used the ready formation of a neutral... 

A benzylidene acetal is a commonly used protective group for 1,2- and 1,3-diols. In the case of a 1,2,3-triol, the 1,3-acetal is the preferred product, in contrast to the acetonide, which gives the 1,2-derivative. The benzylidene acetal has the advantage that it can be removed under neutral conditions by hydrogenolysis or by acid hydrolysis. Benzyl groups and isolated olefins have been hydrogenated in the presence of 1,3-benzylidene acetals. Benzylidene acetals of 1,2-diols are more susceptible to hydrogenolysis than are those of 1,3-diols. In fact, the former can be removed in the presence of the latter. A polymer-bound benzylidene acetal has also been prepared. ... 

We now tum our attention to the C21-C28 fragment 158. Our retrosynthetic analysis of 158 (see Scheme 42) identifies an expedient synthetic pathway that features the union of two chiral pool derived building blocks (161+162) through an Evans asymmetric aldol reaction. Aldehyde 162, the projected electrophile for the aldol reaction, can be crafted in enantiomerically pure form from commercially available 1,3,4,6-di-O-benzylidene-D-mannitol (183) (see Scheme 45). As anticipated, the two free hydroxyls in the latter substance are methylated smoothly upon exposure to several equivalents each of sodium hydride and methyl iodide. Tetraol 184 can then be revealed after hydrogenolysis of both benzylidene acetals. With four free hydroxyl groups, compound 184 could conceivably present differentiation problems nevertheless, it is possible to selectively protect the two primary hydroxyl groups in 184 in... 

Cyclic Acetals Benzyl ethers are obtained from benzylidene acetals on Ni but the product ethers are cleaved about as readily as the acetals.166 The hydrogenolysis of a p-bromobenzylidene derivative was achieved on 10% Pd/C in AcOEt and NaHC03.154... 

Application of hydrogenolysis of benzylidene acetals to 2-amino-2-deoxy sugar derivatives was found to be possible.217 The reductive cleavage of benzyl 4,6-0-benzylidene-2-(benzyloxycarbonyl)amino-2-deoxy-a-D-glucopyranoside (209) gave, in a yield of 50%, a mixture of the 4-O-benzyl (210) and the 6-O-benzyl (211) derivatives. If the re-gioselectivity of this reaction, although notably inferior, was in ac-... 

Vis and Karrer114 have found that treatment of methyl 4,6-O-benzyl-idene-2,3-O-p-tolylsulfonyl-a-D-glucoside (13) with lithium aluminum hydride provides methyl 4,6-0-benzylidene-3-deoxy-a-D-n 6o-hexoside (IS) in good yield. Since hydrogenolysis is possible at both C-2 and C-3 of the two possible 2,3-anhydro derivatives, only the alio isomer (14) could give the 3-deoxy derivative by further reaction with a reagent. Indeed, none of the alternative 2-deoxy derivative was found. This transformation is, un-... 

The hydrogenolysis of methyl 3,5-0-benzylidene-a- and -J3-D-xylofuranoside derivatives gave 5-benzyl ethers as main products. In some cases the attack of the reagent occurred at the ring oxygen of the furanoside skeleton to yield 5-O-benzyl-1 -O-methyl-xylitol derivatives [200]. 

Methyl-,4C D-glucuronate was prepared44 from the D-glucofuran-uronic acid derivative 22 (obtained by benzylidenation of D-glu-curonic acid in the presence of zinc chloride) by treating its sodium salt with methyl-14C iodide, followed by hydrogenolysis of the ester 23 in the presence of palladium-on-carbon the infrared (i.r.) spectrum of the product was identical to that of authentic 2. 

Qeavage of Ai-benzylidene and Af-diphenylmethylene derivatives is performed either by acidolysis (1M HCl) or hydrogenolysis (Hj, Pd/C). 

The 3-methyl ethers of the 4,6-0-benzylidene and 4,6-0-ethylidene derivatives of methyl 2-(A-benzyloxycarbonyl)amino-2-deoxy-a-n-gluco-pyranoside have been prepared by a sequence of reactions paralleling Neuberger s synthesis. The d anomer of the latter compound was obtained directly by methylation of 2-(A-benzyloxycarbonyl)amino-2-deoxy-4,6-0-ethylidene-D-glucose with dimethyl sulfate and alkali. Both anomers were transformed into the free amines by catalytic hydrogenolysis. ... 

In the synthesis of N-acetyl-muramyl-peptides, and obviously of MDP, the most frequently used protected muramyl derivative is 1-O-benzyl-4,6-O-benzylidene-N-acetyl-muramic acid. This allows the mild and possibly simultaneous elimination of the benzylidene group and the benzyl group. Starting from D-glucosamine, either a- or p-benzyl-4,6-0-benzylidene-N-acetyl-muramide can be obtained (Fig. 4) 17, 25, 53). In spite of its slower hydrogenolysis rate 44), the a-isomer which is easier to prepare is generally preferred (55, 44) to the p-isomer 50). 

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