Thiirane, three-membered ring

Three-membered rings containing one sulfur atom are named as thiiranes (1) or thiirenes... 

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. 

The geometric parameters of the three-membered ring sulfones and sulfoxides have been determined via X-ray diffraction techniques and gas-phase microwave spectroscopy. The accumulated data for some selected thiirane and thiirene oxides and dioxides (16-19) as well as for the corresponding thiirane (20) and the acyclic dimethyl sulfone (for the sake of comparison) are given in Table 3, together with the calculated values. 

A unique characteristic feature of the cyclic three-membered ring sulfones and sulfoxides is the dramatic increase in the length of the carbon-carbon single bonds and the carbon-carbon double bonds in the series of thiirane-thiirane oxide-thiirane dioxide (20a -> 16a -> 17a), and thiirene-thiirene oxide-thiirene dioxide (21 -> 18a -> 19b). 

The situation is entirely different in the three-membered ring sulfones and sulfoxides the facile thermolytic elimination of S02 from the former is probably their most distinctive (and dominant) chemical reaction, whereas the loss of SO from the latter characterizes both the thiirane and thiirene series2. 

The formation of alkenes from thiirane dioxides may not be stereospecifically controlled in the presence of a sufficiently strong base and sufficiently acidic protons in the three-membered ring. Under such conditions (essentially those typical for the Ramberg Backlund reaction), epimerization via a carbanion intermediate produces an equilibrium mixture of thiirane dioxides19,99 and consequently a mixture of cis- and trans-alkenes. 

To summarize under favorable conditions the acidity of a-hydrogens facilitates the generation of a-sulfoxy and a-sulfonyl carbanions in thiirane and thiirene oxides and dioxides as in acyclic sulfoxides and sulfones. However, the particular structural constraints of three-membered ring systems may lead not only to different chemical consequences following the formation of the carbanions, but may also provide alternative pathways not available in the case of the acyclic counterparts for hydrogen abstraction in the reaction of bases. 

Actually, thiirane dioxides (17) have so far never been isolated in these reactions cis- and trans-olefins were the main products, and all attempts to obtain the three-membered ring system and prevent the loss of SO 2 failed. Hence, the method can be used only for the in situ formation of intermediates. 

The main result of the thermolysis of the three-membered ring sulfoxides and sulfones is the extrusion of the sulfur monoxide and the sulfur dioxide moieties (Section III.C. I)99 10 5. Only in the presence of a suitably disposed /J-hydrogen does the ordinary sulfoxide-sulfenic acid fragmentation take place in the thiirane oxide series (equation 9). 

The base-induced Ramberg-Backlund rearrangement later initiated extensive mechanistically - and synthetically -oriented investigations, and played a significant role not only with respect to the study of thiirane dioxides (3b), but also contributed substantially to the present state of the art concerning three-membered rings containing sulfur (equation 2). 

The above relationships between the thiiranes (20) and their dioxides (17) are reminiscent of those between cyclopropane and cyclopropanone. The entire phenomena of the C—C bond lengthening and the concomitant C—S bond shortening in the three-membered ring sulfones and sulfoxides can be accounted for in terms of the sulfur 3d-orbital participation and the variation in the donor-acceptor capacities of the S, SO and S02 . The variations of the calculated valence-state orbital energies, together with the corresponding variations of the C—C overlap populations, can be used to understand the discontinuous variations of the C—C and the C—S bond lengths in the series thiiranes -... 

The chemical shifts of the H-methyl groups in thiiranes 31a, 31b and 31c were found to be = 1.59,1.44 and 1.45, respectively. The chemical shifts of the -anti-methyl hydrogens (i.e. those of R ) where found to be (5 = 1.25,1.23 and 1.27 in 32a-c compared with <5= 1.74 and 1.64 for syn-R -hydrogens in 32a and 31c, respectively . The consistency of the deshielding effect in accordance with the position of the -hydrogens in ring sulfoxides is thus apparent. These observations validate the applicability of the S—O anisotropy rule to the three-membered ring system. 

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