The conformation of the title compound (2a) has been determined by X-ray crystallography and by solution 'H n.m.r. studies. The compound crystallises in the triclinic space group P1 and its structure was solved and refined to an R factor of 0.051, from 1 256 observed reflections. In the solid state the molecules possess a distorted trans lactam structure (Am z 30") with a P-helical disulphide bridge.Individual molecules in the crystal are H-bonded between the side chain amide and lactam functions to form a P-pleated sheet array. Assignment of the solution conformation was carried out by comparison of coupling constants with calculated values derived from crystal data, comparison of chemical shifts, and temperature coefficients of the N H resonances in (CD,),SO and CDCI, and by n.0.e. difference measurements. The structure and conformation of (2a) in solution was found t o be similar t o that in the solid state. The sulphone ( 6) was shown to have a similar solution conformation. C(7)-N(71)-C(72) 120.2(12) N(7 1 )-C(72)-0(72) 12 1.4( 14) N(71)-C(72)-C(73) 116.8(13) 0(72)-C(72)-C(73) 121.8( 14) C(72)-C(73)-C(74) 11 1.8(13) C(73)-C(74)-C(75) 121 .O( 13) C(73)-C(74)-C(79) 120.5( 14) C(75)-C(74)-C(79) 1 1 8 3 14) C(74)-C(75)-C(76) 121.6(15) C(75)-C(76)-C(77) 116.6(17) C(76)-C(77)-C(78) 122.1( 17) C(77)-C(78)-C(79) 121.7( 18) C(74)-C(79)-C(78) 119.4( 16)6') N(5')-C(6')-C(7') N(5')-C(6')-0(61') C( 7')-C( 6 ) -0 ( 6 1 ') C(6')-C(7')-C(8') C(6')-C(7')-N(7 1') C(8')-C(7')-N(71') S(1 ')-C(8')-C(7') C(4')-C(4 1 ')-0(4 1 ') C(4')-C(4 1 ')-O(42') O(4 1')-C(4 1 ')-O(42') C(41')-0(42')-C(43') C(7')-N(71')-C(72') N(71')-C(72')-0(72') N( 7 1 ')-C( 72')-C( 73') 0(72')-C(72')-C(73') C( 72')-C( 7 3')-C( 74') C(73')-C(74')-C( 75') C( 73')-c( 74')-C(79') C(75')-C(74')-C(79') C(75')-C( 76)-C( 77') C( 74')-C(75')-C( 76') C(76')-C(77')4(78') C(77')-c(78')-C(79') C(74')-C( 79')-C(78')

Isothiazole-3-and 4-carboxylate esters are readily prepared by 1,3-dipolar cycloaddition of nitrile sulfides, generated by thermal decarboxylation of the corresponding 1,3,4-oxathiazol-2-ones, to acrylate, fumarate and maleate esters, followed by phase-transfermediated hypochlorite oxidation of the resulting 2-isothiazoline cycloadducts.

The periselectivity of the nitrile sulfide cycloaddition to acyl cyanides has been examined. Benzonitrile sulfide, generated by thermal decarboxylation of 5-phenyl-1,3,4-oxathiazol-2-one, reacted exclusively at the cyano group of benzoyl cyanide to afford 5-benzoyl-3-phenyl-1,2,4-thiadiazole. Similar perispecificity was observed with p-toluonitrile sulfide and acetonitrile sulfide, and for cycloaddition of benzonitrile sulfide to 2-furoyl cyanide, acetyl cyanide and 2-oxooctanonitrile. The reactivity of benzoyl cyanide as a dipolarophile was shown to be similar to that of ethyl cyanoformate in a competition experiment which afforded a 51:49 mixture of 5-benzoyl-and 5-ethoxycarbonyl-3-phenyl-1,2,4-thiadiazoles.

No abstract

The conformation of the title compound (IV) which is prepared as outlined in the scheme is determined by X‐ray crystallography (P1; Z=4) and by solution 1H NMR studies.