Diastereomeric Secalonic Acids

Abstract

equilibrium mixture of ( I ) , (2) and (3); compound (5) was rearranged, with a 1,s-homodienyl hydrogen shift [3], into bicyclo[3,3,0]octa-2,7-diene. The photolysis of (3) gave rise to ( 4 ) , (5), and (7)-(10).The formation of (4) and (5) in 22 and 10 yields indicates that they originate from (2) which occurs as an intermediary product. This assumption could be supported by photolysis of 5,8-dideuterocycloocta-1,3,6-triene (3a). Its product of isomerisation (4u) showed a distribution of deuterium which corresponds t o an isomerization (3) + ( 2 ) , by way of a 1,5-hydrogen shift [4]. Compound (7) was forme&, on the other hand, without hydrogen shift: The product (7. ) which was obtained on photolysis of (-?a), carries deuterium only at C-4 and C-8. It is suggested that its formation is due to a rearrangement of the santonine type [5]. The 1,2-homotropilidene (11), which would also be expected, is converted to (8) under the conditions of experiment.( 1 1 )Open-chain 1,3,6-trienes can be isomerized in the same way.Thus 5,5-dimethylhepta-l,3,6-triene is readily rearranged on irradiation into a mixture of cis-and trans-3,3-dimethyl-l,2divinylcyclopropane [6]. The structure of (10) was deduced from infrared and N M R spectra. The formation of (10) is the first example of a photochemically induced Diels-Alder reaction. Received, March 12th, 1964 [Z 693/534 IE] German version: Angew. Chem. 76, 378 (1964) [I] A . C. Cope et al., S e c a l o n i c a c i d , the pale yellow main pigment of ergot (Claviceps purpurea), was originally isolated by Kraft [2] and has since been investigated by several groups of workers [l, 3-61. We found that the crystalline secalonic acid obtained from a number of ergot varieties consists of a mixture of t w o d i a s t e r e o m e r s . Thus, secalonic acid derived from German and Austrian cultured ergot and from Spanish wild ergot could be separated chromatographically into secalonic acid A (RF = 0.42) and secalonic acid B (RF = 0.64) [7,8].On the other hand, secalonic acid from a strongly pigmented Canadian wild ergot contained only the slow-moving com-ponent A. The mass spectra of secalonic acids A and B show the same molecular peaks (at M = 638) and the same fragmentation. Both acids have identical chemical formulae, ultraviolet spectra, functional groups, and exhibit the same reactions on degradation with alkali, but have different melting points RFvalues, and optical rotations (cf. Table 1).They are therefore diastereomers. Table 1. Properties oE the diastereomeric secalonic acids A and 9, C32H30011 (mol. wt. 638.6). M.P. [ " C ] (from CHCI,) 246-247 (decomp.) 254-256 (decomp.) ml: (CHCld