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.

No abstract

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].

On acid‐catalyzed hydrolysis, the tricyclic compounds 2 and 10, incorporating cyclopropyl‐silyl‐ether moieties undergo rearrangement to the cis‐decalones 3 and 7, respectively. Hydrolysis of 2 and 10 in the presence of oxygen leads additionally to the formation of the 1,2‐dioxolan‐3‐ols 9 and 13, respectively, which involves an electron‐transfer oxygenation process as could be demonstrated by photooxygenation of the silyl ether 10 and the cyclopropanol 14 in the presence of 9,10‐dicyanoanthracene. The configurations of 3 and 9 were assigned by X‐ray analysis of the latter compound as well as of the p‐nitrobenzoate 8 of 3.

The substituted 1,2,3,4,4a,9,10,10a-octahydrophenanthrene-4,9-dione 2, synthesized from the cyclohexanone 8 and quinone 11 (Scheme 2), was found by X-ray analysis and 'H-NMR studies to be the isomer with cis-junction of the saturated rings. The cis-fusion could also be determined from the 'H-NMR data of the related compound 17 (Scheme 4 ) , which was previously considered to be trans-fused. In contrary to previous argumentations, the interaction of the C(4)-carbonyl 0-atom of trans-fused octahydrophenanthrenes is more severe with a 5-methoxy than with a 5-methyl substituent.The main problem of a total-synthetic approach to Lysolipin I (l), an interesting antibiotic with an unusual heptacyclic structure [214), is the construction of a highly substituted 9,10-dihydrophenanthrene-4,5-diol a (Scheme 1). Confronted with this problem, we reasoned, that a octahydrophenanthrene-4,9-dione b would be an ideal precursor for a, since, contrarily to the preparation of phenanthrenes and 9,l O-dihydrophenanthrenes, which is seriously hampered by substituents at C(4) and C(5), the preparation of the more flexible octahydro derivatives b by cyclization of (2'-arylcyclo-hexy1)acetic acids c is hardly affected by 4,5-substitution [4] (Scheme 1). The transformation of b to a, involving the introduction of an aroyl substituent at C(6) and of a carboxy group at C(3), OsO, oxidation of a derivative with 9,lO-double bond, and aromatization of ring A, appears to be straightforward. Besides the advantage of introducing chirality in an earlier stage of the synthesis, the additional asymmetric centers of b (C(2), C(4a), and C( 10a)) could be very helpful for relating the asymmetric centers C(9) and C(10) with the remote asymmetric centers of the C(2)-substituent (R') of a.

Die Photolyse der Titelverbindung (IV), die ausgehend von dem Carbinol (Ib) [zugänglich aus Safranal (Ia) mit Methyl‐Li] über die angegebenen Stufen synthetisiert wird, liefert die Produkte (Va)‐(Vc) + (VI)‐(VIII), deren Bildungsmechanismen eingehend diskutiert werden.

The wavelength dependence of the photolysis of 7-methyl-b-ionone ((E)-1) was investigated. Irradiation of (E)-1 with light of 1 > 347 nm leads primarily to (E/Z)-isomerization followed by transformation to the tricyclic enol ether 3 as the only secondary photoproduct. On photolysis of (E)-1 with light of shorter wavelength (A > 280 nm or 1 = 254 nm), however, a series of other products was formed via a ) photocyclization of the dienone chromophore (+ 5), b) photo-enolization (+ S), and c) a 1,5-sigmatropic H-shift (+ (E/Z)-7). For the structure elucidation of the new products, 7-['3C]methyl-b-ionone ((E)-[7-methyl-I3C]-1) was prepared and irradiated furnishing the corresponding "C-labelled photoproducts.