Snthetic Studies on Carotane and Dolastane Type Terpenes: A New Entry to the Total Synthesis of (±)-DauceneViaIntramolecular [2 + 2] Photocycloaddition

Photochemical Approaches to Complex Chemotypes: Applications in Natural Product Synthesis

The use of photochemical transformations is a powerful strategy that allows for the formation of a high degree of molecular complexity from relatively simple building blocks in a single step. A central feature of all light-promoted transformations is the involvement of electronically excited states, generated upon absorption of photons. This produces transient reactive intermediates and significantly alters the reactivity of a chemical compound. The input of energy provided by light thus offers a means to produce strained and unique target compounds that cannot be assembled using thermal protocols. This review aims at highlighting photochemical transformations as a tool for rapidly accessing structurally and stereochemically diverse scaffolds. Synthetic designs based on photochemical transformations have the potential to afford complex polycyclic carbon skeletons with impressive efficiency, which are of high value in total synthesis.

A unified approach to the daucane and sphenolobane bicyclo[5.3.0]decane core: enantioselective total syntheses of daucene, daucenal, epoxydaucenal B, and 14-para-anisoyloxydauc-4,8-diene

Access to the bicyclo[5.3.0]decane core found in the daucane and sphenolobane terpenoids via a key enone intermediate enables the enantioselective total syntheses of daucene, daucenal, epoxydaucenal B, and 14-para-anisoyloxydauc-4,8-diene. Central aspects include a catalytic asymmetric alkylation followed by a ring contraction and ring-closing metathesis to generate the five- and seven-membered rings, respectively.

Photochemical reactions as key steps in natural product synthesis

Photochemical reactions contribute in a significant way to the existing repertoire of carbon-carbon bond-forming reactions by allowing access to exceptional molecular structures that cannot be obtained by conventional means. In this Review, the most important photochemical transformations that have been employed in natural product synthesis are presented. Selected total syntheses are discussed as examples, with particular attention given to the photochemical key step and its stereoselectivity. The structural relationship between the photochemically generated molecule and the natural product is shown, and, where necessary, the consecutive reactions in the synthesis are illustrated and classified.

Tandem intramolecular photocycloaddition-retro-Mannich fragmentation as a route to spiro[pyrrolidine-3,3'-oxindoles]. Total synthesis of (+/-)-coerulescine, (+/-)-horsfiline, (+/-)-elacomine, and (+/-)-6-deoxyelacomine

Irradiation of a tryptamine linked through its side-chain nitrogen to an alkylidene malonate residue results in an intramolecular [2 + 2] cycloaddition to the indole 2,3-double bond. The resultant cyclobutane undergoes spontaneous retro-Mannich fission to produce a spiro[indoline-3,3'-pyrrolenine] with relative configuration defined by the orientation of substituents in the transient cyclobutane. The tandem intramolecular photocycloaddition-retro-Mannich process, abbreviated as TIPCARM, leads to a spiropyrrolidine which is poised to undergo a second retro-Mannich fragmentation that expels the malonate unit and generates transiently an indolenine. The latter undergoes rearrangement to a beta-carboline, which upon brominative oxidation undergoes further rearrangement to an oxindole. With tryptamine as starting material, the entire sequence leads to the alkaloid (+/-)-coerulescine. Starting from 5-methoxytryptamine, a parallel series affords (+/-)-horsfiline. Modification of the malonylidene unit to include an isobutyl substituent at C3 affords a photosubstrate which also undergoes the TIPCARM process. In this case, a 2'-isobutyl-substituted spiro[indoline-3,3'-pyrrolenine] results. This undergoes stereoselective hydride reduction to give a product with relative orientation at the spiro carbon and the new stereocenter bearing the isobutyl appendage corresponding to that of the alkaloid elacomine. From tryptamine, the sequence paralleling that leading to coerulescine and horsfiline terminates at 6-deoxyelacomine, whereas 6-methoxytryptamine as starting material affords (+/-)-elacomine itself.

ent-Daucane and acorane sesquiterpenes from x Cupressocyparis leylandii foliage

One new ent-daucadiene and enantiomers of four previously reported daucadienes or acoradienes were identified in foliage of the hybrid cypress species xCupressocyparis leylandii. The major compound is (+)-dauca-5,8-diene; minor congeners are (-)-dauca-4,8-diene, (-)-dauca-8,11-diene, (-)-acora-3,7(14)-diene and 1S-dauca-4(11),8-diene. None of these compounds is observed in foliage of either of the parent species, Cupressus macrocarpa and Chamaecyparis nootkatensis. However, we have subsequently found them in some specimens of Cupressus macnabiana.

A new and simple approach to chemical complexity. Application to the synthesis of natural products

A new approach to describe the complexity of chemical structures is proposed. This index is simple and can be easily programmed. It derives from Whitlock's index and, despite its empirical character, provides results paralleling those obtained with the mathematical Bertz index. It has been applied to follow the strategic progress of some natural product syntheses.