Iridoids conversion

The facile conversion of carbonyl groups into lactones via cyclobutanones offers many opportunities for synthetic applications considering the importance of butanol-ides in natural products synthesis. The iridoids vividly illustrate this potential. Allamandin (163) 135 c) and its dehydrated relative plumericin (164) 135 d), compounds possessing antifungal, antibacterial, and antitumor activity, pack a great deal of... 

This new reaction was first observed as one step in the conversion of the hexaacetate (1) of aucubin, an iridoid glucoside, into the lactone 3, a possible chiral intermediate to prostaglandins. ... 

The conversion of a bicyclo[2.2.1]octenone derivative to the corresponding bicyclo[3.3.0]octenone, a common intermediate in the total synthesis of several iridoid monoterpenes, was achieved by N.C. Chang et al. The target was obtained by sequential application of the Corey-Chaykovsky epoxidation, Demjanov rearrangement and a photochemical [1,3]-acyl shift. 

Whitesell and Minton have synthesized (- )-xylomollin (408), the only trans-fiised iridoid, from the racemic bicyclic diene 409. Control of the stereochemistry was effected in the first step by addition of the glyoxylate 410. The two products were separated and the major one, 411, was reduced with lithium aluminum hydride. Conversion of the primary alcohol to a methyl group, with concomitant inversion of stereochemistry at the secondary alcohol carbon atom was carried out by protection of the primary alcohol function (fert-butyldimethylsilyl), tosylation of the secondary hydroxyl, then removal of the silyl group with formation of an epoxide with inversion, and reduction (LiEtaBH) of the epoxide. The remaining steps are shown in Scheme 36. It remains to point out that isoxylomoUin (412) was produced preferentially, and is indeed formed from xylomollin (408) slowly in methanolic solution. ... 

G. In Vitro Conversion of Iridoids and Secoiridoids to Monoterpene Alkaloids... 

As noted in the discussion of the in vitro conversion of the iridoids to the monoterpene pyridine alkaloids (see Section III.G), frequently the iridoid corresponding to the alkaloid is a co-metabolite in the isolation process or a direct progenitor, based on the isolation procedure when ammonia is used. Similarly, one can envisage, with the studies on the planned generation of alkaloids from iridoids (see Section III.H), that alkaloids can be produced semisynthetically that may subsequently be found in nature. 

Roby, M. R. and Stermitz, F. R. (1984) Penstemonoside and other iridoids from Castilleja rhexifolia. Conversion of penstemonoside to the pyridine monoterpene alkaloid rhexifoline J Nat Prod 47, 854-857. 

Deoxyloganin (24) has previously been synthesized by Tietze and coworkers, utilizing an intramolecular hetero-Diels-Alder reaction to construct the iridoid core (Scheme 10). The synthesis commenced with conversion of (5)-citronellal (47) to enol ether 48 in seven steps. Knoevenagel condensation of the aldehyde with Meldrum s acid, followed by in situ intramolecular hetero-Diels-Alder reaction afforded pyran 49, with all the carbons required for the natural product core installed. Conversion of 49a, via methanolysis and a reduction/elimination sequence, to lactol acetate 50, was achieved in four steps. Finally, glycosylation and deprotection provided the natural product in a total of 14 steps. 

Formation of Deoxyloganic Acid from Geraniol Formation of 8-e/ i-Deoxyloganin and Related Iridoids Iridoids Derived from 10-Hydroxycitronellol Conversion of Loganin to Secologanin Secologanin as a Precursor for Indole Alkaloids Distribution of Iridoid Monoterpenes Biological Activity... 

Loganin (6) is a key intermediate in the biosynthetic pathway leading to other iridoid monoterpenes as well as complex indole alkaloids. Many of these additional compounds are derived by conversion of this bicyclic iridoid monoter-pene into secologanin (18). The mechanism by which ring cleavage occurs to yield secologanin is not well understood, but apparently the cleavage happens after oxidation of the... 

The intramolecular ene reaction of the 1,6-enyne (2) appears to be retarded by a terminal methyl substituent, but significantly accelerated by an electron-withdrawing substituent, such as the methoxycarbonyl group.These observations are pertinent to the conversion (2) — (3) in a synthetic strategy to the iridoid carbon skeleton. 

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