Thiepins and Dithiins

Since this topic was last reviewed, a book on seven-membered heterocycles containing oxygen and sulphur has been published.  

A number of successful thiepin syntheses has become available. The first monocyclic thiepin has been prepared by cycloaddition at - 30 °C of dimethyl acetylenedicarboxylate to enamines derived from 2,3-dihydrothiophen-3-ones. The adduct (1) rearranged to (2), which rapidly extruded sulphur at room temperature. The same method was applied to the synthesis of substituted benzo[6]thiepins (3) and of 2,3- and 2,7-dihydrothiepins. The structure of a compound, earlier believed to be a thiepin (see Vol. 3, p. 440), has been revised, and it is now considered to be a 2-thiabicyclo[3,2,0]heptadiene, formed by [2 + 2] cycloaddition. 

Benzo[Z)]thiepin (3), and some of its derivatives, have been prepared by the 

Attempted synthesis of [ /]annelated thiepins from heterocyclic dialdehydes and X(CH2COR)a (X = S, SO, or SO R = OMe, Ph, etc.) (cf. ref. lb) has met with only limited success, sulphur-free products being isolated in most cases. 

Interest in the powerfully neuroleptic 10,ll-dihydrodibenzo[6,/]thiepins (6) continues, and some 30 papers and many patents have been published by Protiva s group alone since the last Report. An improvement in the original synthesis of (6) consists of the use of thallium nitrate for the ring expansion 

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In general, these Reports cover the period from April 1974 to March 1976 unless otherwise stated. Due to necessary limitations of space, the topics have, in parts, been treated more summarily than in previous Reports. A review of the chemistry of thiepins and dithiins, omitted from Volume 3, has been included in this volume, but accounts of the chemistry of thiazepines and of heterocyclic compounds of quadricovalent sulphur have been held over to Volume 5. 

Diene moieties, reactive in [2 + 4] additions, can be formed from benzazetines by ring opening to azaxylylenes (Section 5.09.4.2.3). 3,4-Bis(trifluoromethyl)-l,2-dithietene is in equilibrium with hexafluorobutane-2,3-dithione, which adds alkenes to form 2,3-bis-(trifluoromethyl)-l,4-dithiins (Scheme 17 Section 5.15.2.4.6). Systems with more than two conjugated double bonds can react by [6ir + 2ir] processes, which in azepines can compete with the [47t + 27t] reaction (Scheme 18 Section 5.16.3.8.1). Oxepins prefer to react as 47t components, through their oxanorcaradiene isomer, in which the 47r-system is nearly planar (Section 5.17.2.2.5). Thiepins behave similarly (Section 5.17.2.4.4). Nonaromatic heteronins also react in orbital symmetry-controlled [4 + 2] and [8 + 2] cycloadditions (Scheme 19 Section 5.20.3.2.2). 

Formal replacement of one double bond in thiepin (51) by a sulfur atom leads to 1,2- (53) and 1,4-dithiin (54). The latter compound has been synthesized,70 and it is a non-planar, thermally stable molecule with a reactivity widely different from that of aromatic systems. A refined HMO method (Model B)71 has been used successfully to explain known properties of 54, and remarkably good agreement was obtained between the calculated and experimental C—S—C and C—C—S bond angles. For the parameters used (8S =0, pcc = 1.06, pcs = 0.77), the highest occupied orbital is anti-bonding, in agreement with the high... 

Other systems which should be included in this section are the thiepin derivatives 196 and 197 which, if the analogy with six-membered ring systems is pursued, correspond to dithiin and oxathiin (Section III,D, 11). These compounds, together with the S-oxides 198 and 199, and the S,S-dioxides 200 and 201 have been studied by Schlessinger et UV... 

Theoretical Aspects of Organosulphur and Organoselenium Compounds is a review for the period April 1970 to March 1974, with emphasis in this volume on conjugated systems. A review of the chemistry of Thiepins, Dithiins, and Thiazepines has been omitted. A review of these subjects, covering the four-year period April 1972 to March 1976, will appear in Volume 4. Reviewing of all other material covered in this Volume is for the two-year period April 1972 to March 1974. 

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