Pyrroline ring

A number of examples of acid catalyzed ring expansion of acyl and thioacyl azetidines to sbc-membered rings have been reported (B-73MI50903). This type of rearrangement (Scheme 2) is similar to the more general vinylaziridine to pyrroline ring expansion. 

Use of substituted systems has shown that the reaction is stereospecific.300 The groups on C(2) and C(5) of the pyrroline ring rotate in the disrotatory mode on going to product. This stereochemistry is consistent with conservation of orbital symmetry. 

Alike olefins, allenes also undergo palladium mediated addition in the presence of N-H or O-H bonds. Although these reactions show some similarity to Wacker-type processes, from the mechanistic point of view they are quite different. Allenes, such as the cr-aminoallene in 3.69., usually undergo addition with palladium complexes (e.g. carbopalladation in 3.69. and 3.70., or hydropalladation in 3.71.), which leads to the formation of a functionalized allylpalladium complex. Subsequent intramolecular nucleophilic attack by the amino group leads to the closure of the pyrroline ring.87... 

H-5), 7.5-6.8 (unsaturated JS-H), 2.0 (NH) for (iBchl).2,14 The signal of a proton on the meso carbon attached to a pyrroline ring is located at 1 p.p.m. higher frequency than the others partly because of the increased electron density on the position and partly because of the loss of pyrrolic ring current effects. 

The j8 hydrogens of the pyrroline rings are substituted with deuterium under basic conditions (BulOK-BulOD/THF, r.t) without isomerization to other hydroporphyrins. The acidic proton-... 

These low selectivities were improved by changing the amine substituents of the catalysts 34 from diisopropyl/dicyclohexyl to C2-symmetric 2,5-dimethyl-pyrroline ring, leading to two diastereomeric catalysts 35 (Scheme 6.14), which were synthesized in the same way as catalysts 34 [9]. The catalytic competency of 35 was established using standard phase-transfer alkylation conditions to afford the alkylation product with up to 37% ee. Enantioselectivities obtained using diastereomers of the catalyst 35 were different, but not opposite, which suggests that the stereocenters function cooperatively in one diastereomer, but not in the other. 

Generalization of this result to imines of conjugated aldehydes provides a good synthesis of the A2-pyrroline ring. The intermediacy of an ylid is confirmed by trapping with iV-phenylmaleimide, which takes place with excellent stereocontrol.330... 

The results reveal that baker s yeast is a potent source for precursors of 2-acetyl-l-pyrroline. It appears likely that the flavor compound is formed in the yeast cells from proline and dihydroxyacetone phosphate via 1-pyrroline and pyruvaldehyde. This is corroborated by the results of c-labeling experiments which showed that the acetyl group in the Acp stems from a sugar degradation product and that the pyrroline ring was derived from proline. 

As a consequence of the tendency of porphinoids to form an 18 membered 18 7i-electron delocalization circle, NH-groups will be found preferentially at pyrrolic rings trying to avoid the nitrogens of hydrogenated pyrroline rings. 

Many pyrrolizidine alkaloids are metabolized to toxic pyrrole metabolites in the liver by mixed-function oxidases. The structural and chemical features necessary for the formation of these metabolites have been discussed.77 The most important features, in addition to the 3-hydroxymethyl-3-pyrroline system, are steric hindrance to hydrolysis of the ester, lipophilic character (favouring attack by the hepatic microsomal enzymes), and the presence of a conformation that allows preferential oxidation of the pyrroline ring rather than 7V-oxidation. The alkylating activities of a series of these pyrrole derivatives have been examined.78... 

The type-2 systems are the basis of a new type of very potent photochromic compounds. Several routes to this type of system have been developed recently. Incorporation of a A2-pyrroline ring into a more complex molecule has led to a very efficient new class of photochromic compounds (Scheme 2). 

Annelation of steroidal dienamines with substituted phenacyl bromides (7 examples) or with benzenediazonium salts (11 examples) has been shown to lead to the corresponding furano- and indolo-steroids.89 Thus the A3,5-dienamine derived from A4-androstene-3,17-dione reacted with p-bromophenacyl bromide to yield the A5-androstano[3,4-h]furan (199) in 26% yield, and reaction of the same A3,5-dienamine with benzenediazonium fluoroborate at -45 °C led to formation of the hydrazone (200) which underwent Fischer-indole cyclization on treatment with phosphorus oxychloride to produce the A4-androstano[6,7-6]indole (201). The A3,5-dienamine derived from 17/3-acetoxyandrost-4-en-3-one has been converted into the benz[4,5,6]-steroid (202 R1 = Me, R2 = H) by reaction with methyl vinyl ketone and into the analogous benzsteroid (202 R1 = H, R2 = Me) on treatment with crotonal-dehyde.90 A route to the condensed pyrroline ring system (203) has been devised... 

The EPR spectrum of the DMPO H spin adduct (Eq. (20)) consists of a 1 2 2 1 hyperfine quartet (Fig. 16) that arises from the coupling of the unpaired electron spin with the nuclear spins of the N atom (/ = 1) and the p-H atom of the pyrroline ring (7=1/2), which have virtually identical hyperfine coupling constants (flN = a = 14.85 G). 

This sequence has been used to convert a pyrroline ring to useful functional groups with control of stereochemistry. (This reaction is a variation of the oxidation of oxaziridines to nitroso compounds. )... 

An asterisk denotes an atom associated with a pyrroline ring... 

Although the A -pyrroline structure CCI had generally been accepted to represent myosmine, the location of the double bond in the pyrroline ring had never been rigidly demonstrated. A study of the infrared... 

Saponification of 137 with methanolic potassium hydroxide furnished iV -acetylveracintine (138). Biogenetic considerations led to the proposed attachment of the pyrroline ring to C-17. 

The base peak in the mass spectrum at m/e 82 was advanced for a pyrroline ring resulting from the C-20 and C-22 bond fission. The IR spectrum indicated the presence of a hydroxy group and an azomethine double bond. Therefore veracintine was assigned the constitution 20-(2-methyl-l-pyrrolin-5-yl)pregn-5-en-3j8-ol (134). 

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