A Concise Diastereoselective Total Synthesis of α-Ambrinol.

(-)-cis-α-Ambrinol is a natural product present in ambergris, a substance of marine origin that has been highly valued by perfumers. In this paper, we present a new approach to its total synthesis. The starting material is commercially available α-ionone and the key step is an intramolecular Barbier-type cyclization induced by CpTiCl2, an organometallic compound prepared in situ by a CpTiCl3 reduction with Mn.

Fatty acid profiling in the genus Pinus in relation to its chemotaxonomy and nutritional or pharmaceutical properties.

To develop and utilize the oil of Pinus seeds and explore natural resources rich in pinolenic acid (PNLA), twenty-one Pinus taxa were evaluated in a search of Δ5-unsaturated polymethylene-interrupted fatty acids (Δ5-UPIFA)-rich oils. While the fatty acid (FA) composition was determined by GC-FID and GC-MS, NMR of crude oils proved to be a fast method for establishing the ratio between Δ5-UPIFA and total FA. For all analyzed taxa, both the geographical origin and the concentration of total FA in the seeds are provided. PNLA and sciadonic acids occurred in all samples, while taxoleic and bishomopinolenic acids were present in most taxa. PNLA reached a maximum of 28.3% of total FA in P. mugo, and P. koraiensis showed the highest total FA amount (66.8 g/100 g seeds). The previously unanalyzed taxon P. ponderosa var. scopulorum can be considered a new PNLA source (17.1%). Principal Component Analysis showed that the similarities in FA profiles allow the grouping of the various taxa within Pinus subsections and confirmed the differential metabolic activities of Δ5 and Δ9 desaturase enzymes. This study showed that several underutilized Pinus taxa could be developed into renewable woody oil species, and their seeds could be used as raw materials for Δ5-UPIFA-rich oils extraction.

Enantioselective total synthesis of putative dihydrorosefuran, a monoterpene with an unique 2,5-dihydrofuran structure.

An original synthesis of the structure of dihydrorosefuran, a compound allegedly identified in Artemisia pallens and Tagetes mendocina, has been developed. The key steps in the five-step 36% overall yield synthesis are a CpTiIIICl2 mediated Barbier-type allenylation of a linear aldehyde and the formation of a 2,5-dihydrofuran scaffold through a Ag(I)-mediated cyclization. Neither of the reported spectral data for dihydrorosefuran match those of the synthetic product, suggesting that the isolated compound from Tagetes mendocina is in fact the natural product rosiridol, while the real structure of the product from Artemisia pallens remains unknown.

Marine Terpenic Endoperoxides.

Organic extracts of marine invertebrates, mainly sponges, from seas all over the world are well known for their high in vitro anticancer and antibiotic activities which make them promising sources of compounds with potential use as pharmaceutical leads. Most of the structures discovered so far have a peculiar structural feature in common: a 1,2-dioxane ring. This is a highly reactive heterocycle that can be considered as an endoperoxide function. Together with other structural features, this group could be responsible for the strong biological activities of the substances present in the extracts. Numerous research programs have focused on their structural elucidation and total synthesis since the seventies. As a consequence, the number of established chiral centres and the similarity between different naturally occurring substances is increasingly higher. Most of these compounds have a terpenoid nature, mainly diterpene and sesterterpene, with several peculiar structural features, such as the loss of one carbon atom. Although there are many reviews dealing with the occurrence of marine peroxides, their activities, or potential pharmaceutical uses, no one has focused on those having a terpene origin and the endoperoxide function. We present here a comprehensive review of these compounds paying special attention to their structural features and their biological activity.

Stereoselective Barbier-Type Allylations and Propargylations Mediated by CpTiCl3.

CpTiCl2, prepared in situ by manganese reduction of CpTiCl3, is an excellent new system for the Barbier-type allylation and propargylation of carbonyl compounds. It can be used in catalytic amounts when combined with Et3N·HBr/TMSBr, which acts as a regenerating system. The high regio- and stereoselectivity shown by this system makes it useful for prenylation and crotylation processes in the synthesis of natural products.

Ti-catalyzed straightforward synthesis of exocyclic allenes.

Exocyclic allenes constitute useful building blocks in organic synthesis and have recently been identified as key intermediates in the synthesis of natural products. Here the first general method for the most straightforward synthesis of exocyclic allenes reported to date is presented. This method is based on the Barbier-type cyclization of propargyl halides catalyzed by titanium; a safe, abundant, and ecofriendly metal. The reaction proceeds under mild conditions compatible with different functional groups and provides good yields of five-, six-, and seven-membered carbocycles and nitrogen-containing heterocycles bearing an exocyclic allene group. Experimental evidence supporting the proposed reaction mechanism is also provided. Moreover, this procedure can be carried out in an enantioselective manner by using chiral titanocene(III) catalysts. The utility of this method has been proved in the synthesis of the natural alkaloid stemoamide.

Ti-catalyzed homolytic opening of ozonides: a sustainable C-C bond-forming reaction.

The unprecedented homolytic opening of ozonides promoted and catalyzed by titanocene(III) is reported. This novel reaction proceeds at room temperature under neutral, mild conditions compatible with many functional groups and provides carbon radicals suitable to form C-C bonds via both homocoupling and cross-coupling processes. The procedure has been advantageously exploited for the straightforward synthesis of the natural product brittonin A.

Synthesis of pterocarpans.

The total synthesis of natural pterocarpans and analogs is reviewed.

Reaction of benzoxasilocines with aromatic aldehydes: Synthesis of homopterocarpans.

Condensation of 2H-benzo[g][1,2]oxasilocines with aromatic aldehydes in the presence of boron trifluoride affords mixtures of cis/trans 2-phenyl-3-vinylchromans with moderate yields. These can be transformed into homopterocarpans, a synthetic group of substances homologous to the natural isoflavonoid pterocarpans.

Diastereoselective synthesis of 2-aryl-3-vinyl-2,3-dihydrobenzo[b]furans through a Sakurai reaction: a mechanistic proposal.

The condensation of 2,3-dihydrobenzoxasilepins with aromatic aldehydes in the presence of boron trifluoride to form 2,3-dihydrobenzofurans shows a level of diastereoselection which is a function of the electronic nature of the aldehyde and the polarity of the solvent. The study of the mechanism of the reaction demonstrated that it proceeds through a ring-opened allylfluorosilane, which is stable enough to be isolated and characterized.

Silver-catalyzed asymmetric synthesis of 2,3-dihydrobenzofurans: a new chiral synthesis of pterocarpans.

2,3-Dihydrobenzofurans can be diastereoselectively prepared by condensation of aromatic aldehydes with 2,3-dihydrobenzoxasilepines under the catalysis of Ag(I) complexes, and in the presence of a source of fluoride ion. The application of this strategy by using chiral catalysts leads to a new enantioselective total synthesis of natural cis-pterocarpans and their trans isomers. Through this method, the first enantioselective total synthesis of the antifungal agent (-)-pterocarpin was achieved. In addition, a new entry into the heteroaromatic system of 2,5-dihydrobenzoxepine is also presented.

A concise and diastereoselective total synthesis of cis and trans-pterocarpans.

A new strategy for the diastereoselective and convergent synthesis of pterocarpans which is able to control the relative stereochemistry of the molecule through allylation of aromatic aldehydes with cyclic allylsiloxanes is described.