Mo(CO) 3 (CNt-Bu) 3 (MoBI 3 ) was found to be a suitable catalyst for the regioselective hydrostannation of several types of alkynes, especially propargyl alcohol derivatives, affording preferentially the a-stannylated products. If propargylic acetates are used, the stannylated allylic acetates produced are suitable substrates for Pd-catalyzed allylic alkylations. Allenyl carbinols also undergo regioselective hydrostannation in the presence of MoBI 3 , because the allenyl carbinols are more reactive than alkynes, and therefore milder reaction conditions are possible. Allylstannanes are formed preferentially, which can easily be converted into allyl iodides or vinyl epoxides.

In the molybdenum-catalyzed hydrostannation of alkynes the addition mode of tributyltin hydride has a strong effect on the outcome of the reaction. Slow addition increases the yield, even if the amount of tributyltin hydride is reduced to only one equivalent. Microwave irradiation speeds up the reaction dramatically, sometimes at the cost of regioselectivity.From a synthetic point of view, transition-metal-catalyzed reactions are extremely interesting and important, not least because many of these reactions proceed under very mild conditions. 1 In general, they also tolerate a wide variety of functional groups and are therefore suitable for the synthesis of complex molecules. Of the various transition metals used, palladium reigns supreme and is commonly used for cyclizations, allylic alkylations, cross-couplings, and many other important reactions. 2 In particular, Stilletype couplings are extremely popular 3 using aryl or vinyl stannanes as organometallic coupling partners. The latter can easily be obtained by hydrostannation of alkynes, 4 which can be carried out under different reaction conditions. Besides a radical pathway, a palladium-catalyzed version of the hydrostannation has also been developed during the last few years. The major drawback of both methods results from difficulties in controlling the regioselectivity of the tributyltin hydride addition to unsymmetrical alkynes. A comparison of these two protocols shows that the transition-metal-catalyzed version has the advantage of a clean cis addition, which results from the mechanism of the reaction. Therefore, only two products are formed during this process, though radical tributyltin hydride additions generate E/Z mixtures of the corresponding stannylated alkenes. Most examples described so far employ palladium complexes as catalysts. However, other transition metals, such as rhodium complexes can also be used. 5 Guibé et al. described an application of a molybdenum catalyst, 6 and recently, Chiu et al. applied a copper complex to this type of reaction. 7

Substituted 4-piperidones are easily obtained from alkynes via molybdenum-catalyzed regioselective hydrostannation and subsequent Stille-coupling of the vinyl stannanes obtained with a,b-unsaturated acyl chlorides. The resulting divinylketones can undergo a double 1,4-addition of amines giving rise to the required 4-piperidones. The best results are obtained if the Stille couplings and the 1,4-additions are combined in a one-pot procedure.Piperidines are an important class of heterocycles, found widely in nature, for example in the large group of piperidine alkaloids. 1 Probably the most famous representative is the poison of the hemlock Conium maculatum, coniin (1), 2 but many other derivatives are highly interesting from a pharmaceutical point of view as well. Hydroxylated piperidines such as isofagomine (2) 3 and related structures are important glycosidase inhibitors, 4 while piperidones such as 3 inhibit human platelet aggregation ( Figure 1). 5 Figure 1Natural and pharmaceutically interesting piperidine alkaloids Therefore, substituted 4-piperidones are highly interesting scaffolds for the synthesis of drug-like molecules. Besides classical approaches such as Dieckmann condensations, 6 hetero-Diels-Alder reactions 7 and cyclizations via 1,4-addition of amines towards vinylic ketones 8 are suitable for the synthesis of these heterocycles.Our group is interested in transition metal catalyzed hydrostannations and the application of this protocol to natural product and heterocycle synthesis. 9 We found that Mo(CO) 3 (CNt-Bu) 3 (MoBl 3 ) is a highly efficient catalyst for the regioselective hydrostannation of terminal alkynes, 10 giving rise to internal vinyl stannanes. 11 These and similar mixed carbonyl/isonitrile complexes were used previously by Trost et al. for allylic alkylations, 12 and we found that MoBl 3 was superior in the hydrostannation reaction to the generally used palladium complexes, 13 especially with respect to regioselectivity. The internal vinyl stannanes can subsequently be subjected to Stille couplings, which makes this approach interesting from a combinatorial point of view. For example, 4-piperidinones should be accessible in three steps from alkynes via regioselective hydrostannation, Stille coupling with a,bunsaturated acyl halides, and subsequent double 1,4-addition of amines (Scheme 1). Scheme 1 Retrosynthetic analysis of substituted 4-piperidonesTo prove this synthetic protocol, we subjected protected propargyl amines 4 to molybdenum-catalyzed hydrostannations (Scheme 2). Under standard reaction conditions, three equivalents of Bu 3 SnH, as reported previously, furnished vinyl stannane 5 in 70-87%. To improve the yield and obtain more reproducible results, we carefully varied the reaction conditions and found that the best results were obtained if only 1.1 equivalents of Bu 3 SnH were added slowly over a period of six to seven hours via syringe pump at room temperature. Under these conditions, 5 was obtained in 95% yield in very pure form [b-product/ g-(E)-product/g-(Z)-product, 93:...