Synthesis of cyclopentanones via reaction of furan chromium carbene complexes with alkynes

Construction of the Benzindenoazepine Skeleton via Cyclopentannulation of Fischer Aminocarbene Complexes: Total Synthesis of Bulgaramine

A total synthesis of bulgaramine has been accomplished with a longest linear sequence of eight steps and an overall yield of 23% from commercially available 3,4-dimethoxyphenethyl alcohol. An intramolecular cyclopentannulation reaction of a Fischer aminocarbene complex provided the key step and occurred under significantly milder conditions and in higher yields than those of other reported examples of this reaction type. The reaction solvent was a critical factor in the cyclopentannulation reaction, with measurable amounts of the desired product observed only when THF was utilized. The product yield could be further enhanced by the addition of two-electron donor ligands, demonstrating the first example of this effect on the thermal reaction of aminocarbene complexes with alkynes.

Binuclear biscarbene complexes of furan

Carbene complexes of chromium and tungsten with a bridging furan substituent were synthesized from lithiated furan precursors and metal hexacarbonyls. The binuclear biscarbene complexes [(CO)5M{C(OEt)-C4H2O-C(OEt)}M'(CO)5](M = M'= Cr (3), W (4)) were obtained as well as the corresponding monocarbene complexes [M{C(OEt)-C(4)H3O}(CO)5](M = Cr (1), W (2)). A method of protecting the carbene moiety during the metal acylate stage was used to increase not only the yields of the binuclear Fischer biscarbene complexes 3 and 4 but to establish a method to synthesize analogous mixed heterobinuclear carbene complexes (M = W, M'= Cr (5)) in high yields. The binuclear biscarbene complexes 3 and 5 were reacted with 3-hexyne and yielded the corresponding benzannulated monocarbene complexes [M{C(OEt)-C14H17O3}(CO)5](M = Cr (6), W(7)). Complex 5 reacted regioselectively with the benzannulation reaction occurring at the chromium-carbene centre. The major products from refluxing 3 in the presence of [Pd(PPh3)4] were a monocarbene-ester complex [Cr{C(OEt)-C4H2O-C(O)OEt}(CO)5](8), the 2,5-diester of furan (9) and a carbene-carbene coupled olefin EtOC(O)-C4H2O-C(OEt)=C(OEt)-C4H2O-C(O)OEt (10). X-Ray structure analysis of 4 and 6 confirmed the molecular structures of the compounds in the solid state and aspects of electron conjugation between the transition metals and the furan substituents in the carbene ligands were investigated.

Fischer Carbene Complexes as Chemical Multitalents: The Incredible Range of Products from Carbenepentacarbonylmetal alpha,beta-Unsaturated Complexes

The metal carbene complexes, discovered by E. O. Fischer at the start of the 1960s and carrying his name, have since proved themselves to be irreplaceable building blocks for organic synthesis. In particular, since the discovery of the Dötz reaction, a formal cycloaddition of Fischer alpha,beta-unsaturated carbene complexes to alkynes with CO insertion, this area of chemistry has become increasingly interesting to organic chemists. In spite of the considerable diversity of reactions performed with these complexes, proper selection of substrates and careful adjustment of the reaction conditions have allowed, in most cases the perfectly selective preparation of individual compounds of this enormous range of products. The spectrum of new successes begins with the conventional Diels-Alder reaction of alkynylcarbene complexes and the formal regioselective [3+2] cycloaddition of alkenylcarbene complexes to alkynes. It extends much further, however, from cascade reactions with the formation of oligofunctional and oligocyclic products of impressive molecular complexity to complex, formal [3+6] cocyclizations in which six bonds are formed in a single operational step. Beyond doubt, the methodological arsenal of preparative organic chemistry cannot be imagined any more without the valuable transformations of the Fischer carbene complexes; it only remains to be seen whether one or other of the numerous new types of cocyclization products of these complexes can establish itself as a lead structure in the search for biologically active compounds.