A very effective strategy has been devised for the synthesis of 3-substituted pyrroles based on the use of the triisopropylsilyl (TIPS) moiety as a sterically demanding nitrogen substituent to obstruct the attack of electrophilic reagents at the a positions. 1 -(Triisopropylsilyl) pyrrole (1) undergoes highly preferential kinetic electrophilic substitution at the ß position with a variety of electrophiles (Br+, I+, N02+, RCO+, etc.) and fluoride ion induced desilylation of the products provides the corresponding 3-substituted pyrroles in good overall yields. Competitive trifluoroacetylation experiments demonstrate that substitution of TIPS-pyrrole at the a positions is decelerated by a factor of >104, vs pyrrole at the same sites, without affecting reactivity at the ß positions. l-(Triisopropylsilyl)-3-bromopyrrole ( 2) is readily converted into the 3-lithio compound 44 by bromine-lithium interchange with alkyllithium reagents. This previously unavailable, formal equivalent of 3-lithiopyrrole is itself an excellent source of a wide range of /3-substituted pyrroles, many of which would not be directly preparable from 1.TIPS-pyrrole can be 3,4-dihalogenated and these compounds undergo sequential halogen-metal interchange trapping reactions. This process is exemplified by an efficient, three-step synthesis of the antibiotic verrucarin E (63) from the dibromo compound 5. * Contribution no. 792.

The title compounds (E/Z)-7 were prepared in 66% overall yield by reaction of P-ionone ((E)-1) with lithium dimethylcuprate, trapping of the intermediate enolate with benzeneselenenyl bromide and oxidation with H202. Analogously, (E/Z)-7methyl-a-ionone ((E/Z)-12) was obtained in 65 % yield from a-ionone ((E)-11). In, 71 *-Excitation (A > 347 nm, pentane) of (E)-7 causes rapid (E/Z)-isomerization and subsequent reaction of (2)-7 to 15 (66%). The formation of 15 is explained by twisting of the dienone chromophore due to repulsive interaction of the 7-CH3-group with the CH,-groups of the cyclohexene ring. On the other hand, irradiation (A > 347 nm, Et20) of (E)-7 in the presence of acid leads to (2)-7 (5%) and to the novel compound 16 (88 %). Scheme 1)').4. Structure of the Compounds. -Only the most relevant spectral data are discussed herein; full data and assignment of the NMR signals are presented in the Exper. Part. 7-Methyl-ionones (E/Z)-7 and (E/Z)-12.The assignment of the enone double bond configuration is based on the chemical shift of the 'H-NMR signal of the 7-CH3-group. Due to the anisotropy effect of the carbonyl group in (E)-7 and (E)-12, the d of the 7-CH3-group is shifted downfield to 2.12 and 2.09 ppm, respectively, whereas the corresponding signals of (2)-7 and (Z)-12 appear at 1.89 and 1.69 ppm, respectively. Additionally, due to the same effect, the signal of the doubly allylic H-atom of ( 2 ) -1 2 at 4.10 ppm is shifted cu. 2 ppm downfield in comparison with that of (E)-12.Further evidence for the above assignment was obtained by oxydation of (E)-and ( 2 ) -7 to y, 6-epoxyenones which showed different reactivity. On thermolysis (160"), the y, 6-epoxyenone derived from (2)-7 underwent the characteristic (2)-epoxyenone/furan rearrangement, whereas the y, 8-epoxyenone derived from (E)-7 proved to the stable [12] [13]. Furthermore, on treatment with 80% H2S0,, (2)-7-methyl-a-ionone ((2)-12) was transformed to a ca. 1 : 1 mixture of the corresponding 8-ionone compound (2)-7 and the aforementioned bicyclic dienol ether 16.Tricyclic E n d Ether 15. The structure was derived from the spectral data (see Exper. Part). The presence of the enol ether moiety which is evidenced by an IR band at 1667 cm-' was proven by methanolysis of 15 leading to the acetal 17 (73%, see Scheme 3 ) . Furthermore, ozonolysis of 15 in MeOH afforded the formyl acetate 18 (38%) and its dimethyl acetal 19 (34%). Scheme 3 17 18*) Chromatography fractions containing (2)-4 and 5 in various ratios gradually changed to a ca. 1 : 3 equilibrium mixture of (2)-4 and 5.

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Photo-oxygenation of (E)-7-methyl-B-ionone ((E)-1) and (E)-Z-rncthyl-B-ionone ((4-2) gave rise to the formation of the hydroperoxy-enones (E)-10 and (E)-15, respectively, which, in part, underwent intramolecular epoxidation to the hydroxy-epoxy-ketones 11 and 16, respectively. The product distribution of the photo-oxidation of (Z)-1 shows a marked influence of the skewed ground-state conformation of the dienone chromophore. Thus, singlet oxygen ('0,) was added to C(y) of the dienone chromophore leading to the spirocyclic peroxy-hemi-acetall2 and to the endoperoxide 13. In addition. the tricyclic peroxide 14 was formed as a new type of product via primary addition of '0, to C(S) of the dienone chromophore. The structure of 14 was established by X-ray crystal-structure analysis of the hemiacetal22. ') 150th Communication: [l].