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16α,17-and 15α,16-Epoxy derivatives of diterpenoid steviol having ent-kaurane structure were found for the first time to undergo Wagner-Meerwein rearrangement in alkaline medium or by the action of boron trifluoride-diethyl ether complex to give products with ent-beyerane structure. The geometric parameters of steviol 16α,17-and 15α,16-epoxides were determined by X-ray analysis.In the recent years, studies on biotransformations of diterpenoid steviol (I, 13-hydroxy-ent-kaur-16-en-19-oic acid) having ent-kaurane structure and its epoxy derivative II have been reported [1][2][3][4]. Epoxide hydrolases isolated from various microorganisms are universal biocatalysts for asymmetric hydrolysis of epoxides [4], and alcohols thus formed exhibit diverse biological activity, in particular hormone [3] and antihyperglycemic [2]. As concerns chemical modification of steviol epoxides II and V, their hydrolysis in the presence of mineral [5] and Lewis acids [6] and reduction with lithium tetrahydridoaluminate [6] were reported (Schemes 1, 2). In most cases, these reactions are accompanied by the Wagner-Meerwein rearrangement with formation of products having ent-beyerane (isosteviol) skeleton, i.e., compounds IV (Scheme 1) [5,6], VI, and VII [6] (Scheme 2). In the present article we describe opening of the oxirane rings in steviol 16α,17-and 15α,16-epoxides II and V by the action of Lewis acid (boron trifluoride-diethyl ether complex BF 3 · Et 2 O).Epoxide II was synthesized by oxidation of steviol (I) with m-chloroperoxybenzoic acid by analogy to the procedure reported in [7]. Compound II was assigned

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O-(Trimethylsiloxy)alkyl S-trimethyl-silyl aryldithiophosphonates 7a-d were obtained by the reaction of 2,4-diaryl-1,3,2,4-dithiadiphosphetane-2,4-disulfides 5a,b with disilyl derivatives of glycols 6a,c and salicyl alcohol 6b. The reactions of mixed O,S-bis(trimethylsilyl) 2, 4-di(3,5-di-tert-butyl-4-hydroxyphenyl)dithiophosphonate 1 and S-silyl aryldithiophosphonates 7a,b with S,S-diethyldithiodiphenylgermane 2, dichlorodiphenylgermane 8a, and dichlorodiphenylstannane 8b were studied. The structure of hexaphenyl-2,4,6,1,3,5-