Three-carbon annulations synthesis

There exists also a synthesis of cyclopentadecanone (VII/81) and ( )-mus-cone, based on a three-carbon annulation of cyclic ketones followed by the regioselective radical cleavage of the zero bridge of the so formed bicyclic system [44], The synthesis of cyclopentadecanone is summarized in Scheme VII/16. The cyclization of VII/78 to the bicyclic alcohol VII/79 proceeds best (94 % yield) with samarium diiodide in the presence of hexamethylphosphoric acid triamide and tetrahydrofuran [45], The oxidative cleavage of VII/79 to the ring expanded product VII/80, was performed by treatment with mercury(II)-oxide and iodine in benzene, followed by irradiation with a 100 Watt high pressure mercury arc. Tributyltinhydride made the de-iodination possible. 

The intramolecular carbene addition from the diazoketone (58), leading to (59), is a key step in a new synthesis of hirsutene (60) described by Hudlicky and co-workers (Scheme 9). In a second new approach to hirsutene, Greene has applied his so-called three-carbon annulation procedure, based on the diazomethane-induced ring expansion of a-chlorobutanones, announced last year (Scheme 9). 

A three-component coumarin synthesis involves the Pd-catalysed coupling of o-iodophenols with alkynes and subsequent insertion of carbon monoxide. With internal alkynes, pyridine is the crucial base for successful annulation the regioselectivity with unsymmetrically substituted alkynes is only moderate (Scheme 43) . 

Allenylsilanes serve as valuable three-carbon components in a general [3+2] annulation method for the synthesis of five-membered rings. A variety of general synthetic approaches to allenylsilanes have recently been developed and a number of specialized routes to various specific functionalized derivatives are available as well. The present procedure... 

The procedure described here serves to illustrate a general [3+2] annulation method for the synthesis of cyclopentane derivatives. A unique feature of this one-step annulation is its capacity to generate regio-specifically five-raembered rings substituted at each position, functionally equipped for further synthetic elaboration. As formulated in the following equation, the reaction proceeds with remarkably high stereoselectivity via the effective suprafacial addition of the three-carbon allene component to an electron-deficient olefin ("allenophile"). ... 

The known preparative methods for the synthesis of imidazo[l, 5-a]pyrimidines prior to 1983 have been reviewed <84CHEC-I(5)607>. This ring system has been sparsely reported in the literature, most syntheses reported involve annulation of a 4-aminoimidazole with a three-carbon unit. Some more recent methods are described below. 

Three-Carbon Synthon for [3 + 2] Annulations. Danheiser and co-workers have exploited allenylsilanes as the three-carbon components in a [3 + 2] annulation strategy for the synthesis of a variety of five-membered carbocycles and heterocycles. The pathway by which a typical annulation proceeds is shown in eq 6. Reaction of the 2-carbon component (the allenophile ) at C-3 of the allenylsilane is followed by rapid rearrangement of the silicon-stabilized vinyl cation. Ring closure then affords the five-membered product. 

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