Oxirane ring system

This chapter will deal exclusively with three-membered rings containing the hetero atoms O, S and N, and fused to the steroid skeleton. Because of the conformational requirements in steroids, not all of the usual methods of synthesis of three-membered rings are applicable to the fused ring system. For the synthesis of steroids to which an aziridine, oxirane or thiirane is attached either in the side chain or at a ring position but not directly fused to the nucleus, the methods discussed in this chapter, as well as others, are applicable. 

Formation of oxiranes on the sterically more hindered side of the steroid ring system is usually carried out via /raw -halohydrins which afford oxiranes on treatment with base (c -Halohydrins yield ketones on exposure to base). Two general methods are available for the synthesis of tm s-halohydrins (1) the reduction of a-halo ketones and (2) the addition of a hypohalous acid to unsaturated steroids. 

Five-membered ring systems can be obtained from hetero-l,3-dienes on reaction with oxiranes and thiiranes. To avoid competition from a possible 1,4-addition, the nucleophilic attack of the terminal heteroatom of the diene has to be sterically or electronically hindered by incorporation of the heteroatom into... 

Oxiranes exhibit 1,3 [e,n] capacity. Therefore, seven-membered ring systems can be synthesized on reaction with hetero-1,3-dienes. The reaction is catalyzed by 4-dimethylaminopyridine. On catalysis with boron trifluonde, the regioche-mistry is reversed [263] (equation 58). 

These polymers are based on the three-membered heterocyclic system either as the epoxy or oxirane ring (1.3). 

Addition of benzotriazole to l-phenyl-2-aroylacetylenes gives a,/3-unsaturated ketones 221 in high yields. By treatment with dimethylsulfonium ylide, ketones 221 are converted to epoxides 222, Opening of the oxirane ring and electrophilic attack of the obtained tertiary carbocation on N-2 of the benzotriazole system leads to betaines 223 that consecutively eliminate formaldehyde to give triazapentalenes 224 (Scheme 28) <2004ARK(iii)109>. 

The deprotonation of meso oxiranes 144 and 145 by the system alkyllithium/diamine leads to the enantiomeric ketones (S)- and (R)-146 (Scheme 63). Due to the general propensity of these systems to achieve deprotonation at the R carbon of the oxirane ring, it was then concluded that this rearrangement follows exclusively a 1,2-H shift. 

Thermochemical measurements on the oxepin-benzene oxide system are unavailable. However, based upon experimental observations it would appear that the oxepin tautomer is generally more thermally stable than the benzene oxide as a result of the additional ring strain present in the oxirane ring. 

In connection with the oxirane ring opening, the isomerization of oxirane 6 to ketone 9 in an MeCN—LiClO —(Pt) and a CH Clj—Et NClO —(Pt) system has been reported and was explained on the basis of an electron-transfer chain mechanism via 7 and 8 However, 6 is actually converted to 9 under the preelectrolysis conditions so that the transformation may be explained in terms of an EGA catalyzed reaction... 

In the oxirane ring opening reaction (Eq. (2)), it is possible to prepare the ketone 5, the acetonide 10, and the allylic alcohol 11 selectively by controlling the reactivity of the intermediate carbenium ion 12 under some specified reaction conditions. In order to prepare 5 preferentially, a high concentration of perchlorate ion must be employed in CH2CI2, ClCHjCH Cl, or THF. In this system, an incipient carbenium ion 12 would be associated with perchlorate ion and stabilized so that the deprotona-... 

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