Pentalenic acid synthesis

Intramolecular [2 + 2] photocycloadditions of 3-alkenylcyclopent-2-enones represent the key step in the total synthesis of various natural products, e.g. pentalenolactone 6-methyl ester19 and pentalenic acid,20 as well as in the preparation of strained fenestranes21 and laurenene.22 Similarly, the photoisomerization of 3-alkenylcyclohexcnoncs to tricyclo[6.3.0.01-6]undecanones are steps in the syntheses of tricyclic sesquiterpene isocomene23 and precursors to members of the acorane family.24... 

The effect of substituents at the a-carbon of the enone on the stereoselectivity was examined on compounds 270, with the stereogenic center located at the alkenyl side chain. Crimmins and DeLoach122 found that the stereoselectivity encountered upon irradiation of 270 depends on the degree of steric hindrance associated with the ester group linked to the double bond. The ratio of the two epimeric centers at the C-9 position varied from 13 1 (R = Me) to 17 1 (R = Et), then 20 1 (R = i-Pr). These results demonstrate that steric effects play an important role in controlling the stereofacial selectivity in these and related systems. Fragmentation of the photoproduced four-membered ring and simple transformations afforded synthesis of ( )-pentalene 274, (i)-pentalenic acid 275 and (i)-deoxypentalenic acid 276 (Scheme 59). 

Oxidation reactions of this nature are common in the literature. For example, selenium dioxide in refluxing etiumolic solution brought about the allylic oxidative rearrangement geranyl acetate, which was further functionalized in a synthesis of the norsesquiterpenoid gytinidal (equation 46). This trans formation was also used in a total synthesis of phytol. Similarly, an a, -unsaturated aldehyde was obtained undm similar conditions in studies of a synthesis of pentalenic acid derivatives (equation 47). ... 

The pentalane class of sesquiterpenoids has received substantial attention in the past year from the standpoints of structural elucidation, biosynthesis, and synthesis. Two new metabolites of this class are pentalenic acid (298) and pentalenolactone H (299). Both these compounds have a secondary hydroxyl function adjacent to the gem-dimethyl group and are thus potential precursors of pentalenolactone (300) in which one of these methyl groups has undergone a 1,2-migration. Cane and Rossi " have now identified a further metabolite of a Streptomyces strain which has been named pentalenolactone E (301) and is now... 

Transannular cyclization can be induced by acid-mediated activation of a,/J-unsaturated carbonyl groups. Treatment of an a./J-unsaturated ketone with formic acid/boron trifluoridc-di-ethyl ether complex produces the tricycle 1423. Ketone 14 is a precursor in the pentalene 8 synthesis (Prins reaction analog). Transannular Prins cyclizations are described in Section 1.5.5.2.2.2.26-27. 

A new synthesis of pentalenic acid (273) with virtually complete stereo- and regio-control uses Michael addition of the carbanion of the allylic phosphonate (272).Phosphonate-based olefination of suitably substituted cinnamaldehydes has been used to prepare a number of (E,E)-l,4-diarylbuta-1,3-dienes (274) with properties suitable for use in liquid crystal display devices. 139 The meso-form (276) of a tetrahydrotetrabenzocyclododecene has been synthesized by bis-alkylation of the bis-ylide (275) (276) isomerizes to the racemic form on heating. 1 11... 

Rearrangement of the carbenes generated by photoelimination of nitrogen from cyclic diazo-ketones is frequently accompanied by ring-contraction. This process has found extensive use in synthesis recent examples include the synthesis of the [7]paracyclophane ring system,6 4,8-dihydrodibenzo[cRing-contraction was not observed on irradiation of the unstable... 

In several recent communications (1-4) we have reported preliminary results of our attempts at the synthesis of organometallic tt -complexes of the pentalene system. The key step involved solvolysis, in acid solution, of the alcohols l, 3 and 5 to obtain the metal it -complexes of the pentalenylium, methylpentalenylium and benzopentalenylium cations, 2, 4 and 6 respectively. 

---