Dedicated to Dr. Otto Isler on the occasion of his 80th birthday (23.1V.90) Starting from the readily available, optically active (4R)-hydroxy-2,2,6-trimethylcyclohexanone (Z), a new technical synthesis of (3R,3'R)-zeaxanthin is described. According to a completely new C,+C2+C4 = C,, scheme, the ketone 2 was protected, ethynylated with Li-acetylide, and the C, ,-intermediate 6 was acetylated, followed by dehydration. The product 10 was protected, deprotonated, and subsequently reacted with methyl vinyl ketone to provide the C,,-propargylate 13. Reduction in situ of 13 with Vitride yielded the olefinic C,,-alcohol 11 which was transformed into the known C,,-Wittig salt 3. A double Wittig reaction of this salt with the C,,-dialdehyde 4 afforded nature-identical zeaxanthin (1).Introduction. -In the foregoing publication [ 11, we described a new technical synthesis of (3R,3'R)-zeaxanthin (1) starting from the optically active hydroxy-ketone 2 [2] (cf. Scheme). Using this C,-building block and the vitamin-A intermediate (E)-3-methylpent-2-en-4-yn-1-01 (C,-synthon [3]), we prepared the C,,-ally1 alcohol 11 which was converted in three additional steps into the known olefinic C,,-Wittig salt 3 [l] [4-61. Subsequent 'double Wittig reaction' of 3 with the C,,-dialdehyde 4 [7] provided (3 R,3'R)-zeaxanthin (1). SchemeHELVETICA CHIMICA ACTA ~ Vol. 73 (1990) 869In this publication, we describe a completely new approach to the known C,, salt 3 [l] again from ketone 2 but applying a new C,+C,+C, building scheme. In comparison with the former synthesis [ 11, the overall yield of 3 from 2 was increased from 43 YO up to 72 YO.Furthermore, the new approach avoids such problematical steps as the dehydration and hydrogenation of only moderately stable CIS-intermediates.Results and Discussion. -The OH group of 2 was first protected in situ with isopropenyl methyl ether, and the resulting THF solution of the IPM-ketone 5 was then treated with 2 equiv. of freshly prepared Li-acetylide in liquid ammonia (Scheme). After an acidic workup, the crude diol6 [8] [9] was obtained in almost quantitative yield with a purity of more than 98 YO. GLC analysis of this material revealed the presence of ca. 3 YO of the other C,-diastereoisomer [lo]. After acetylation of this crude material with Ac,O in pyridine or in the presence of 1% DMAP, the resulting monoacetate 7 [9] was used directly without purification in the following dehydration step. According to a previously described method on a similar vitamin-A intermediate [I 11 [12], we used as a catalyst 10 mol-YO of CuSO, in boiling o-xylene. The catalyst could be easily removed afterwards by simple filtration and recycled. A detailed investigation of this dehydration step showed that, contrary to [I I] [12], no reaction took place in boiling toluene, and we were not able to find any other suitable catalyst. All the common catalysts (Brmstedor Lewis acids etc.) gave rise to complex mixtures in which the major products were derived from hydration of the triple bond (cf Rupe reaction).Effic...

Dedicated to Dr. Otfo Isler on the occasion of his 80th birthday (23.1V.90) Starting from the readily available, optically active (4R)-4-hydroxy-2,2,6-trimethylcyclohexanone (I), a new technical synthesis of (3R,3'R)-zeaxanthin is described. According to a 2(C9 + C,) + C,, = C,, construction scheme, the ketone 1 was first transformed with (E)-3-methylpent-2-en-4-yn-l-o1(5) into a CIS-intermediate which,by a three-step sequence, could be converted into the known olefinic CI5-Wittig salt 4. Optimized conditions for the final Wittig reaction of4 with the Clo-dialdehyde 3 are discussed. Based on 1, the overall yield of the entire technical process is ca. 40 %.

3,3,6,9,9‐Pentamethyl‐2,10‐diaza‐bicyclo[4.4.0]‐1‐decen and some of its derivatives A simple synthesis for the bicyclic amidine 1 (Scheme 3) is described. This base and the salts which were prepared from it show solubility characteristics which make the amidine a potentially useful reagent for salt formation of carboxylic acids and related proton complexes of bidentate ligands. Among the derivatives made from 1 are the sterically strongly hindered N‐alkylated amidines 11, 12 and 14 (Scheme 5), as well as the stable crystalline N1‐oxidoamidine‐N2‐oxyl radical 2 (Scheme 6). The ability of the latter to serve as a paramagnetic chelating ligand for metal ions is illustrated by the preparation of a corresponding nickel(II) complex. The radical is also a source for the α‐nitronyl‐nitrosonium cation 4 which shows in its reactivity towards conjugated dienes and olefines some of the expected resemblance to singlet oxygen.

No abstract

A New Access to 2,6,6-Trimethylcyclohexa-2,4dienone from COxoisophorone 2,6,6-Trimethylcyclohexa-2,4-dienone (l), a versatile starting material for the preparation of some carotenoids and several natural products, was efliciently (73 YO yield) prepared from oxoisophorone 3. After conversion of 3 to the alcohol 4 or the acetate 5, H 2 0 was eliminated (4-1) under acidic distillative conditions, whereas AcOH could be eliminated (5-+1) under Pd(0) catalysis.