Benzylidene effect

Seeking to emulate the benzylidene effect in p-mannosylation [68] (see Section 4.2.6), David Crich and Baolin Wu [87] and Hiromune Ando and coworkers [88] prepared 5-.V,7-(9-oxazinone-protecled thiosialosides 69 and 70 (Fig. 5.5). Unfortunately, both groups found this class of donors to be moderately p-selective under a variety of activating conditions and even in the presence of acetonitrile. 

If the general mechanism of Scheme 6 is accepted then it follows that the explanation for the benzylidene effect can be found in the effect of this group on the equilibrium constant K2, or more specifically in a reduction of K2-... 

Oxidative procedures have been utilized for the synthesis of both monocyclic five-membered heterocycles and their ring-fused analogs, although the ease of synthesis of the precursors for the latter ring closures results in wider application of this procedure. A variety of oxidizing agents have been used and the conversion of the benzylidene hydrazidines (221) into the 4-arylamino-l,2,4-triazole (222) was effected with mercury(II) oxide (77BCJ953). 

Regioselective benzoylation of methyl 4,6-0-benzylidene-o -galactopy-ranoside can be effected by phase-transfer catalysis (BzCl, Bu4N Cl, 40% NaOH, PhH, 69% yield of 2-benzoate BzCl, Bu4N CP, 40% NaOH, HMPA, 62% yield of 3-benzoate)/... 

As with the benzylidene group a variety of methods shown below have been veloped to effect cleavage of one of the two C—O bonds in this acetal. 

Formation of an 11-methoxybenzyl ether occurs in satisfactory yield during the preparation of a 17,21 -benzylidene derivative [e.ff., (26)]. The usefulness of this ether and of related mixed acetals in removing the inhibitory effect of the 11 j5-hydroxyl group on the 20-ketone reduction has been reported. ... 

In this type of cleavage reaction, it appears that the axial benzoate is the preferred product. If water is excluded from the reaction, a bromo benzoate is obtained.The highly oxidizing medium of 2,2 -bipyridinium chlorochromate and MCPBA in CH2CI2 at rt for 36 h effects a similar conversion of benzylidene acetals to hydroxy benzoates in 25-72% yield. ... 

In about 5-10 minutes a clear solution resulted, whereupon slow crystallization occurred and the temperature rose to about 6°-7°C. The crystallization was permitted to continue overnight at 5°C, and the very fine precipitate was then isolated by centrifugation and in the centrifuge washed with water, ethanol, and ether, yielding the dihydrate of DL-seryl-(2,3,4-trihydroxy-benzylidene) hydrazide hydrochloride, which melted at 134°-136°C and was poorly soluble in cold water, but very readily dissolved in hot water. The condensation was also effected in absolute ethanol yielding the anhydrous form of the hydrazone, which melted at 225°-228°C. 

In 212 cc of water are mixed 21.2 grams (0.112 mol) of N-(benzylidene)-3-amino-2-oxa-zolidone, 8.93 grams of concentrated sulfuric acid, and 30.1 grams (0.124 mol) of 5-ni-tro-2-furaldehyde diacetate. This mixture is heated to effect the hydrolysis of N-(benzy-lidene)-3-amino-2-oxazolidone, steam distillation of the benzaldehyde and hydrolysis of 5-nitro-2-furaldehyde diacetate. Approximately IV2 hours are required for this reaction to take place. When the bulk of the benzaldehyde has been removed, 50 cc of 99% isopropanol are added, the reaction mixture is refluxed a short time, and the crystals of N-(5-nitro-2-furfurylidene)-3-amino-2-oxazolidone are filtered from the hot suspension. The product is washed with water and isopropanol and dried a yield of 23.3 grams, 92.8% based on N-(benzylidene)-3-amino-2-oxazolidone of MP 254° to 256°C is obtained, according to U.S. Patent 2,759,931. 

Allyltantalum-mediated allylation of aldimines was reported by Bhuyan et al. in 1993 these workers compared tantalum and bismuth organometallic reagents over a series of transformations. Up to 60% yield was obtained for tantalum-mediated allylation (benzylidene aniline as electrophile). Bismuth reagents offered better yields in the cases compared but, in all cases, a remarkable effect for the addition of Bu NBr was observed.186... 

Incorporation of the carboxylic acid group into the substrate also had an effect on the stereochemistry of the Alder-ene products. Trost and Gelling60 observed diastereoselectivity in the palladium-catalyzed cycloisomerization of 1,7-enynes when the reactions were conducted in the presence of A,A-bis(benzylidene)ethylene diamine (BBEDA, Figure 2). They were able to synthesize substituted cyclohexanes possessing vicinal (Equation (53)) and... 

This cyclization is possible even when a quaternary center is formed in such reactions, N,N -bis(benzylidene)ethylenediamine, Cf,H5HC=N(CH2)2 N=CHC6H, (6), can be more effective than P(C6H5)a as the ligand.2... 

The sulfoxide method was employed in the direct synthesis of a P-l,2-man-nooctaose (Scheme 4.60) [362-364]. The synthesis of a p-mannosyl phosphoiso-prenoid illustrates the possibility of employing even such weak nucleophiles as phosphates (Scheme 4.61) [365]. Both syntheses rely on the presence of 4,6-0-benzylidene acetal, and its effect on the covalent triflate-contact ion-pair equilibrium [366,367], to influence the stereochemistry of the glycosylation process [295,323],... 

The rings in l,4 3,6-dianhydro-D-sorbitol have different stabilities. Montgomery and Wiggins66 treated the anhydride with hydrochloric acid under pressure and isolated some l,6-dichloro-l,6-didesoxy-D-sorbitol and much 6-chloro-6-desoxy-l,4-anhydro-D-sorbitol, both isolated as their benzylidene derivatives. No l-chloro-l-desoxy-3,6-anhydro-D-sorbitol was found. This experiment demonstrates that the 1,4-anhydro ring is more stable than the 3,6-ring in this substance. This is in agreement with the fact that partial dehydration of D-sorbitol in the 1,4-position can be preferentially effected and 1,4-anhydro-D-sorbitol isolated. 

A very remote secondary H/D isotope effect has been measured for the 2 + 2-cycloaddition of TCNE to 2,7-dimethylocta-2,fran -4,6-triene. The reaction of nitric oxide with iV-benzylidene-4-methoxyaniline to produce 4-methoxybenzenediazonium nitrate and benzaldehyde is thought to proceed via a 2 + 2-cycloaddition between nitric oxide and the imine double bond. A novel mechanism for the stepwise dimerization of the parent silaethylene to 1,3-disilacyclobutane involves a low-barrier [1,2]-sigmatropic shift. Density functional, correlated ab initio calculations, and frontier MO analysis support a concerted 2 + 2-pathway for the addition of SO3 to alkenes. " The enone cycloaddition reactions of dienones and quinones have been reviewed. The 2 + 2-photocycloadditions of homochiral 2(5H)-furanones to vinylene carbonate are highly diastereoisomeric. ... 

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