Total synthesis of (.+-.)-kempene 2

Enantioselective Total Synthesis of the Diterpenes Kempene‐2, Kempene‐1, and 3‐epi‐Kempene‐1 from the Defense Secretion of Higher Termites

“…The diterpenes (+)-kempene-1 (2) and (+)-3-epi-kempene-1 (3) were obtained in 24 steps each. Owing to the domino metathesis strategy, the efficiency of the route developed here is considerably higher than that of the reported synthesis of rac-1 [6] and can be increased further by recycling of the undesired diastereomers of 11.…”

“…[4,5] The synthesis of these exceptional natural products has proven to be problematic. So far, only the total synthesis of racemic kempene-2 (rac-1) has been reported, [6] whereas a number of other synthetic studies [7] have not yet led to the desired kempanes. Recently, we achieved the enantioselective synthesis of 4-desmethyl-3a-hydroxy-15-rippertene (5), a close analogue of rippertene 4, which was isolated from Nasutitermes rippertii and Nasutitermes ephratae; an intramolecular Diels-Alder reaction was the key transformation in the synthesis.…”

“…However, in contrast to the present work, a McMurry reaction served as the key step to generate the seven-membered ring of the target molecule in moderate yield. [6] After reduction of the electron-deficient olefin in 7 under equilibrating conditions, [6] the pure stereoisomer 11 was isolated in 31 % yield. Three additional diastereomers of 11 were isolated (68 % overall yield) and could partially be converted to 11 by treating again with zinc and acetic acid.…”

Enantioselective Synthesis of the Hydroazulene Core of 3α‐Hydroxy‐15‐rippertene

“…[2] Due to their complex architecture, the tetracyclic termite diterpenes are attractive synthetic targets. So far, the total synthesis of a single naturally occurring kempane, kempene-2, [3] has been achieved in racemic form, and many advanced synthetic investigations directed to this class of natural products have been reported. [4,5] We recently reinitiated our studies toward the enantioselective total synthesis of the diterpene 3α-hydroxy-15-rippertene (1) that has been isolated from the defense secretion of the higher termites Nasutitermes rippertii and Nasutitermes ephratae by Prestwich [6] ( Figure 1).…”

The McMurry Coupling and Related Reactions

“…259 The tetracyclic diterpene kempene-2 (123) is synthesized by the cyclization of a dialdehyde to form a seven-membered ring at the final stage. 260 A tricyclic system involving an eight-membered ring is constructed in the syntheses of the ABC system of taxol, 261 taxane triene 124, which comprises the full and stereochemically correct carbon framework of natural taxusin, 262 and the ceroplastin nucleus. 263,264 The ten-membered-ring sesquiterpenes, bicyclogermacrene (125) 265 and its stereoisomer, lepidozene, 265,266 dihydrogermacrene, 267 and helminthogermacrene 268 are prepared through the low-valent titaniumpromoted cyclization.…”