The preparative value of the I-(phenylsulfonyl) N-blocking and directing group for the synthesis of 3-acylpyrroles has been further evaluated. Acetylation and benzoylation are strongly regiospecific and give good yields. However, the regiospecificity is not general and other substitution reactions give mixtures of 2-and 3-substitution or even mostly 2-substitution. Friedel and Crafts rert-butylation gives 3-rerr-butyl-I-(phenylsulfonyl)pyrrole and provides a useful route to rert-butylpyrrole, but ethylation and isopropylation give mixtures. Acylations of 2-and 3-alkyl-I-(phenylsulfonyl)pyrroles show little evidence of useful regiospeeifieity.

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The Vilsmeier–Haack intermediates formed from pyrrole and from 1-methylpyrrole may be acylated under normal Friedel–Crafts conditions. Hydrolytic work-up then gives 4-acylpyrrole-2-carboxaldehydes in good yield. The methoxide/methanol treatment of the 4-trichloroacetylated intermediate leads to methyl 2-formylpyrrole-4-carboxylate. These are all "one-pot" syntheses from pyrrole. The formyl group has been removed from several of the products thus affording some 3-acylpyrroles in two operations.

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A synthetic strategy has been developed which enables pyrrole to be used as the starting material for the synthesis of 2,3,4-trisubstituted pyrroles of the type used to construct porphyrins. The total synthesis of porphobilinogen has been accomplished as a demonstration of the applicability of the approach.

Kinetic data for the first-order ring opening of the dihydro-intermediates, formed on the photocyclisation of several styrylnaphthalene analogues, are presented and some aspects of their electronic spectra are discussed. The photocyclodehydrogenation of 1,2-di-(2-naphthyl)ethylene to benzo [g,h,i]perylene proceeded via dibenzo[c,g]phenanthrene. Photocyclodehydrogenation of 2-methyl-I -(2,4,6-trimethylstyryl) naphthalene, 2-methyl-1 -styrylnaphthalene, I -( 2,4,6-trimethylstyryI)naphthalene, and 1 -( 2,6-dichlorostyryl) naphthalene yielded the expected chrysene derivatives. 1 -Bromo-2-styrylnaphthalene and 2-(2.4.6-trimethylstyryl) naphthalene photocyclised to benzo [clphenanthrene and 2,4-dimethylbenzo[c] phenanthrene respectively. No migration of substituents was observed. The effect of electron impact on these styrylnaphthalenes correlates with their photochemical behaviour. The U.V. and l H n.m.r. spectra of the styrylnaphthalenes are discussed. Iatermediates in Some Photocyclodehydrogenation Reactions.-In 1950, Parker and Spoerri isolated phenanthrene (111) on irradiation of stilbene (I).2 Since then this basic reaction has been exploited as a very useful synthetic route to polycyclic aromatic and heterocyclic ring ~ysterns.~ Unfortunately the mechanism of the that this colour disappears slowly when the irradiating source is removed or when the solution is heated.4 Only 3= A @-8 \ /photocyclodehydrogenation of cis-stilbene analogues has not received as much attention.

stilbene (styrylpyridines) . A preliminary Communication reported the photocyclisation of 3-stilbazole, and of, contrary to the findings of Mallory and his c o -w o r k e r ~, ~~~ 2and 4-stilbazole.

Chlorosulfonyl isocyanate (CSI) reacts not only with pyrrole itself but with pyrroles having powerful electron-withdrawing groups in the 2-position. Suitable choice of electron-withdrawing groups allows selective 4-substitution of the pyrrole ring. The N-chlorosulfonyl amides formed are readily converted to the corresponding nitriles. The procedure has been used to provide syntheses of pyrrole-3-carbonitrile and its 1-methyl homologue.