Rh(I)-catalyzed formal [6 + 2] cycloaddition of allenal 6 having an alkyne or alkene in a tether proceeded smoothly, giving 5-8- and 6-8-fused bicyclic ketone derivatives 7 in good to excellent yields. It was also found that cyclization of enantiomerically enriched (S)-6a (94% ee) gave cyclic ketone derivative (S)-7a in high yield with reasonable chirality transfer (86% ee). This result indicates that this cyclization proceeds through stereoselective formation of rhodacycle H' followed by insertion of a multiple bond.
A method for synthesizing chiral oxazolidinone scaffolds from readily available oxabicyclic alkenes is described. The reaction utilizes a domino sequence of Rh(I)-catalyzed asymmetric ring-opening (ARO) with sodium cyanate as a novel nucleophile followed by intramolecular cyclization to generate oxazolidinone products in excellent enantioselectivities (trans stereochemistry).
Eight-membered carbocyclic compounds are widely found in natural products that have unique medical and biological activities. [1] Transition-metal-catalyzed [m+n] and/or [m+n+o] cycloadditions (e.g., [4+4], [6+2], and [4+2+2]) are the most promising strategies for the construction of polycyclic eight-membered-ring compounds. [2,3] However, the construction of a simple but functionalized monocyclic eightmembered carbocyclic system is still difficult even when using transition-metal-catalyzed cycloadditions, and only a few examples have so far been reported. [4] Herein we report Rh I -catalyzed intermolecular [6+2] cycloadditions of 4-allenals and alkynes to give functionalized monocyclic eightmembered-ring compounds. [4f, 5-8] We recently reported a Rh I -catalyzed intramolecular [6+2] cycloaddition of 4-allenals with tethered alkynes and alkenes (Scheme 1). [5c] In this reaction, the rhodacycle A is initially formed through hydroacylation [9] of the 4-allenal moiety of 1 followed by insertion into a C À C mutiple bond in the tether to afford bicyclic eight-membered-ring compound 2.We envisaged that if this intramolecular [6+2] cycloaddition could be expanded to an intermolecular reaction between 4-allenal 3 and alkyne 4, monocyclic octanone derivative 5 would be obtained (Scheme 2). [10] However, the application of the intramolecular reaction to an intermolecular version is generally difficult because of unfavorable entropy and the high probability of side reactions (e.g., formation of 6 through hydroacylation of allenal 3 [5c] and formation of 7 by trimerization of alkyne 4).To examine the feasibility of the plan, the cyclization of 4allenal 3 a with terminal alkyne 4 a in the presence of various Rh I complexes was initially investigated (Table 1). The use of [Rh(IMes)(cod)]ClO 4 , which is the most effective for the above-mentioned intramolecular cyclization (Scheme 1), afforded the desired eight-membered ring 5 aa in 61 % yield along with six-membered ring 8 aa in 19 % yield (entry 1). [11] It was found that [Rh(SIMes)(cod)]ClO 4 was also effective in this intermolecular reaction, and the cyclic compound 5 aa was produced selectively in 68 % yield (entry 2). Lowering the reaction temperature from room temperature to 0 8C improved the yield of the eight-membered-ring compound 5 aa up to 83 % (entry 3). Furthermore, the catalyst loading could be reduced to 2 mol % under similar reaction conditions, thereby giving 5 aa in 84 % yield (entry 4). On the other hand, [RhCl(PPh 3 ) 3 ] and [Rh(dppe)]ClO 4 did not promote the desired reaction at all, and the starting material 3 a was recovered in 69 % and 78 % yield, respectively (entries 5 and 6).Encouraged by these results, the cyclization of 4-allenal 3 a with various terminal alkynes 4 was examined ( Table 2). Cyclization of 3 a with terminal alkynes 4 b, 4 c, and 4 d, having Scheme 1. Rh I -catalyzed intramolecular [6+2] cycloaddition. L = ligand. Scheme 2. Plan for intermolecular [6+2] cycloaddition.
Rhodium(I)-catalyzed cyclization of allenynes with a tethered carbonyl group was investigated. An unusual insertion of a CO bond into the C(sp(2) )-rhodium bond of a rhodacycle intermediate occurs via a highly strained transition state. Direct reductive elimination from the obtained rhodacyle intermediate proceeds to give a tricyclic product containing an 8-oxabicyclo[3.2.1]octane skeleton, while β-hydride elimination from the same intermediate gives products that contain fused five- and seven-membered rings in high yields.
Rhodium(I)-catalyzed
enantioselective hydroacylation of 4-allenals
was found to proceed smoothly, giving six-membered ketones in good
yields (up to 84% yield) with high enantiomeric excess (up to 96%
ee) even from racemic allenes as substrates. Mechanistic studies revealed
that racemization of the allene moiety in the substrate would occur
via a dynamic kinetic resolution (DKR) process during the hydroacylation.
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