Acetals reductive ring opening

Section 1.1.1.3.1.1.2.1.). Similarly, the reductive ring opening of cyclopropyl ketones, e.g., 8, with lithium in ammonia affords specific enolates20,21. These enolates can, of course, be used for selective alkylation. Furthermore, enol acetates, e.g., 10, are regiospecifically obtained by nucleophilic ring opening of cyclopropyl ketones21. 

P. J. Garegg, J. Hultbeig, and S. Wallin, A novel reductive ring-opening of carbohydrate benzylidene acetals, Carbokydr. Res. 108 97 (1982). 

M, Ek, P. J. Garegg, H. Hultberg, and S. Oscarson, Reductive ring openings of carbohydrate benzylidene acetals using borane-trimethylamine and aluminium chloride. RegioSelectivity and solvent dependence, J. Carbokydr. Chem. 2 305 (1983). 

For the synthesis of the block, 36 was glycosylated with 34 using the inverse procedure [10] affording a high yield of disaccharide (37) (Scheme 5.6). Reductive ring-opening of the benzylidene acetal gave the 4-OH derivative (38), which was coupled with the imidate (35) to yield the trisaccharide block (32). 

L. Jiang and T.-H. Chan, Borane/Bu2BOTf A mild reagent for the regioselective reductive ring opening of benzylidene acetals in carbohydrates, Tetrahedron Lett., 39 (1998) 355-358. 

R. Johansson and B. Samuelsson, Regioselective reductive ring-opening of 4-methoxybenzylidene acetals of hexopyranosides. Access to a novel protecting-group strategy,/. Chem. Soc., Perkin Trans., 1 (1984) 2371-2374. 

C. C. Wang, S. Y. Luo, C. R. Shie, and S.-C. Hung, Metal trifluoromethanesulfonate-catalyzed regioselective borane-reductive ring opening of benzylidene acetals A concise synthesis of 1,4-dideoxy-l,4-imino-L-xylitol, Org. Lett., 4 (2002) 847-849. 

Oxidative cyclization of o-aminoacylbenzenes is a much less common process for the synthesis of anthranils than the reductive cyclizations discussed in the previous section, mainly because the o-aminoketones are in many instances best prepared by reductive ring opening of anthranils (see Section III,C,3). Nevertheless, a few examples have been recorded. Oxidation of o-aminobenzophenones to the nitroketones using peroxytrifluoro-acetic acid, permaleic acid, or persulfuric acid proceed via the 3-phenyl-anthranil which occasionally is isolable.168 Caro s acid is also a useful oxidant.169... 

Regioselectivity is important for reductive ring opening of cyclic acetals as differentiation of the two hydroxyl groups in a diol system is often desired in synthesis. The fact that the combination of Et,SiH with TfOH and PhBCT operate in the opposite sense on... 

Cleavage of chiral acetals. ( 4/ ,6/ )-4,6-Dimethyl-l,3-dioxanes undergo reductive ring opening to give mainly the product with (5)- configuration at the newly formed stereogenic center. 

P. I. Givegg and R Hultberg, A novel, reductive ring-opening of carbohydrate benzylidmie acetals, with unusual regioaelectivity, Carbohydr. Res. 95.-C10 (1981). 

Acetal cleavage Interestingly, the regioselectivity in the reductive ring opening of 4.6-O-benzylidene glucose and glucosamine derivatives by borane-dimethylamine-boron trifluoride etherate, is dependent on the solvent used. 

The Padwa group has also extended this methodology to provide a route to the hexacycUc framework of the kopsifoline alkaloids [64—66]. Compound 119, prepared by treating the precursor diazo imide 118 with rhodium (11) acetate, was isolated as a single diastereomer in 98% yield (Scheme 30). This intermediate was then converted to the hydroxy ester 120 via a reductive ring-opening/cyclization sequence in 68% yield. Treatment of 120 with Smla in HMPA furnished the ketoester 121 in 93%... 

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