Cleavage of Sapogenin Terminal Rings With Lithium Aluminum Hydride

Abstract

4) 9 modification of the apparatus of H. Theotell and A. AkcaJon (Arkiu. Kemi Mincvai. Gcol., 16A, No. 8, 1943), designed and kindly made avsilahle to ua by Profearor C. A. Knight,

Regioselective Ring Opening of 1,3‐Dioxane‐Type Acetals in Carbohydrates

“…The action of LiAlH 4 on acetals and ketals was first investigated in 1951, when the regioselective ring opening of the acetal in diosgenin, a phytosteroid, was reported by Doukas and Fontaine. [56] Inspired by this novel transformation, several other chemists investigated the reductive cleavage of acetals and ketals with LiAlH 4 in combination with AlCl 3 as a Lewis acid. [57][58][59][60] Although the use of these reagents gives rise to harsh reaction conditions and is incompatible with a variety of moieties that are reduction sensitive, for instance esters, nitro groups, epoxides and azides, this methodology has found its fair amount of use.…”

“…Among the various reagents that have been probed for regioselective ring opening of acetals, the aluminium hydride family can be considered archetypical. The action of LiAlH 4 on acetals and ketals was first investigated in 1951, when the regioselective ring opening of the acetal in diosgenin, a phytosteroid, was reported by Doukas and Fontaine . Inspired by this novel transformation, several other chemists investigated the reductive cleavage of acetals and ketals with LiAlH 4 in combination with AlCl 3 as a Lewis acid .…”

Triphenylcarbenium Tetrafluoroborate as an Efficient Catalyst in the Regioselective Reductive Ring Opening of Benzylidene Acetals of Carbohydrates

“…In literature, there are several methods described for the regioselective ring opening of benzylidene acetals. [2] The first example of this transformation was reported by Doukas and Fontaine in 1951, where they describe the opening of the 4,6-acetal of diosgenin using LiAlH 4 and HCl. [3] Later, numerous efficient reagents and conditions have been explored for ring opening of simple benzylidene acetals and for the regioselective ring opening of 4,6-O-benzylidene acetals in hexapyranosides.…”

“…[2] The first example of this transformation was reported by Doukas and Fontaine in 1951, where they describe the opening of the 4,6-acetal of diosgenin using LiAlH 4 and HCl. [3] Later, numerous efficient reagents and conditions have been explored for ring opening of simple benzylidene acetals and for the regioselective ring opening of 4,6-O-benzylidene acetals in hexapyranosides. [2,4,5] Most commonly, ring opening is catalyzed by Lewis acids such as BF 3 .Et 2 O, [6] triflates, [6,7] and AlCl 3 [8] in combination with hydrides such as BH 3 -THF, [6,7] LAH [4a] and DIBALÀ H. [9] Furthermore, the effect of solvent on regioselectivity utilizing the reagent combination BH 3 -NMe 3 -AlCl 3 , have been studied.…”

Regioselective reductive openings of 4,6-benzylidene acetals: synthetic and mechanistic aspects

“…They thus subjected diosgenin (1), a steroidal precursor in the semisynthesis of progesteron and other steroids, to lithium aluminum hydride and, in the presence of hydrogen chloride gas in anhydrous ether the acetal opened to give dihydrodiosgenin (Scheme 1). 1 A couple of years later Eliel and Rerick reasoned that the reagent in the reduction might be lithium aluminum hydride-aluminum chloride, and subsequently subjected a number of acetals (e.g., 3, 5, and 7) to these conditions and thus isolated the corresponding ethers in good yields (Scheme 2). 2,3 The best yields were observed using a ratio of AlCl 3 In 1969 Bhattacharjee and Gorin transferred the use of reductive openings of benzylidene acetals to carbohydrate chemistry.…”