Conversion of 2‐bromomethylstyrene 22 and benzocyclobutenyl carbamate 28 to the benzophenanthridine alkaloids (±)‐chelidonine (1, five steps, 25% from 28) and to (±)‐norchelidonine (2, six steps, 24% from 28) are described. The key step 29 → 31 involves a highly regio‐ and stereocontrolled intramolecular Diels‐Alder reaction of the (E)‐quinodimethane 30.
SummaryThe pyrrolidines 2 and 10 were obtained by thermal ene-reactions at +70" and + 180" from the (Z)-4-aza-1,6-diene I and from the (E)-4-aza-1,6-diene 9 in the ratios of 75: 25 and 50: 50, respectively. On the other hand, these cyclizations proceeded readily in the presence of diethylaluminum chloride at -78" and -35" giving in high yield the trans-pyrrolidine 2 from 1 with 100% and from 9 with 89% diastereoselectivity .Recently, we have reported a simple synthesis of the racemic amino acid 3 by the sequence 1 + 2 -+ 3 (Scheme I ) [ 11. In conjunction with our systematic studies of intramolecular ene reactions [2] we felt it worthwhile to re-examine the key step 1 -2 in more detail. One important question was the extent to which the unusual 75% diastereoselectivity in favour of the trans-product 2') depends on the enophile geometry in 1.was easily accessible viu the previously described preparation of the (Z)-acrylate 6 (J(AB)= 12 Hz) [3] (treatment of the cis-P-chloroacrylate 4Thermal cyclization of 1 furnished a 3:l mixture of 2 and its cis-isomer 10 as we found by 'H-NMR. analysis of the cyclization mixture (vide infru) and of the aminoacid mixture obtained together with (k )-3.
SummaryThe monocyclic amino diacid (+)-a-allokainic acid 1 has been prepared enantioselectively from the ester of cis-P-chloroacrylic acid and (-)-8-phenylmenthol by a series of four synthetic operations in over 15% yield. The crucial step is the intramolecular 'ene-type' reaction of the (a-diene 4 which on treatment with a mild Lewis acid undergoes a highly accelerated, dia-and enantiostereoselective cyclization to give the pyrrolidines 6 and 7 in a ratio of 95: 5 (Scheme 3). Subsequent ester hydrolysis regenerates the auxiliary chiral alcohol. Similar cyclization of the (E)-diene 5 furnished a 15: 85 mixture of 6 and 7 showing an efficient reversal of the optical induction by variation of the enoate geometry.(+)-a-Allokainic acid, isolated from Digenea simplex AG., has been assigned structure 1 on the basis of chemical, chiroptic and X-ray evidence [l]. In the preceding communication we have described stereospecific routes to the (Z)-and (a-dienes 2 (R = Et) and their efficient diastereoselective cyclization to the truns- We only know about one study allowing recyclization of the chirality directing group which deals with the Lewis-acid catalyzed addition of menthylglyoxalate to 1-pentene; the corresponding adducts were obtained in only modest optical yields in spite of wide variations of temperature, solvent and catalyst [4]. IR., 'H-NMR. (CDCI3) and MS. are in full agreement with the assigned structure.
2,
Die Synthese der Titelverbindung (VIII) erfolgt ausgehend von der cis‐Chloracrylsäure (Ia) über die Stufen (IIIa), (Va) und (VI), das so unter den angegebenen Bedingungen stereoselektiv gewonnen werden kann.
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