1.3- Dipolar electrocyclization

The reaction of a thiocarbonyl and a-oxodiazo compound that leads to 1,3-oxathioles has been rationalized by a 1,5-dipolar electrocyclization reaction (178). It was suggested that an intermediate thiocarbonyl yhde bearing a C=0 function at the a-position (extended dipole) was first formed. Due to the low reactivity of a-oxodiazo compounds, these reactions were carried out at elevated temperatures or in the presence of rhodium acetate as the catalyst. In some cases, catalysis by LiC104 was also reported (77-80). 

The metal-catalyzed formation of 2,3-dihydrothiophene derivatives via a 1,5-dipolar electrocyclization has been reported by Hamaguchi et al. (124). For example, the Rh2(OAc)4-catalyzed reaction of vinyldiazo compound 159 (R = Ph) with xanthione (160) produced the spirocychc drhydrothiophene 161. In contrast, when 159 containing a methyl group (R = Me) was used, thiirane 162 was the sole product (Scheme 5.48). This result was rationalized by the selective formation of an intermediate thiocarbonyl yhde 163 with (Z)- and ( )-configuration, respectively. 

Cyclization of a thiocarbonyl ylide with the C=C-bond of an aromatic ring was observed in the reaction of aryl biphenyl-2-yl ketones with di(tosyl)diazomethane in the presence of Rh2(OAc)4 (189). In the case where the aryl ring contains a 4-methoxy group, benzo[c]thiophene (164) was the only product formed. In contrast, when the aryl ring consists of a 2,4,6-trimethylphenyl group, compounds 165 and 166 were produced. It would seem that after 1,5-dipolar electrocyclization of the intermediate thiocarbonyl ylide occurs, aromatization then takes place by elimination of toluenesulfinic acid or methyl toluenesulfinate. 

An attempt to ringclose the free acid 8 to the benzoxazinone 3 with the help of acetic anhydride led to an unexpected result If 8 (or preferable 3) is heated in acetic anhydride the oxadiazolyl benzoxazine 12 is obtained. However, the outcome of this reaction is not surprising since R. Huisgen [68AG359] and others, have shown, that the acylation of tetrazoles give 1,3,4-oxadiazoles via N-acylnitrilimines (such as 11) by 1,5-dipolar electrocyclic ringclosure. 

The action of chloramine T on the acylhydra-zones 79 (R, R = aryl or 2-furyl) generates nitrile imines 80, which undergo a 1,5-dipolar electrocyclization to the 1,3,4-oxadiazoles 81 (94SC1879). 

The decomposition of dimethyl diazomalonate with Rh2(OAc)4 in toluene at 50 °C in the presence of 1 leads to two products 15 and 16, which are formed via an intermediate thiocarbonyl yUde (eq 8). Whereas 1,3-dipolar electrocyclization affords a thiirane which spontaneously extrudes sulfur to give 15, the competitive 1,5-dipolar electrocyclization yields 1,3-oxathiole 16. ... 

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