A catalytic asymmetric synthesis of 8-oxabicyclo[3.2.1]octane derivatives was achieved through the [3+2]-cycloaddition of the platinum-containing carbonyl ylides generated from acyclic gamma,delta-ynones on treatment with 10 mol % of PtCl(2)-Walphos and AgSbF(6). Synthetically useful 8-oxabicyclo[3.2.1]octane derivatives were obtained in good yields and mostly in over 90% ee's.

Transition-metal-containing zwitterionic intermediates generated through the electrophilic activation of alkynes toward nucleophilic attack by a carbonyl oxygen atom or imino nitrogen atom can be used in addition reactions with nucleophiles or cycloaddition reactions with alkenes or alkynes to prepare synthetically useful heterocyclic compounds. [1][2][3][4][5] However, in almost all cases, it is necessary to employ rigid aromatic substrates for the effective generation of the zwitterionic species. Herein, we report a catalytic method for the generation of nonconjugated platinum ( [6] was found to be very effective. Thus, the treatment of 1 a with platinum(II) chloride (10 mol %) in the presence of n-butyl vinyl ether at room temperature gave the 8-oxabicyclo[3.2.1]octane derivative 2 a in 78 % yield as a single diastereoisomer with the butoxy substituent in an exo orientation.We propose the following mechanism for the formation of 2 a: The electrophilic activation of the alkyne moiety in 1 a by platinum(II) chloride induces the attack of the carbonyl oxygen atom onto the alkyne in a 6-endo manner to generate the platinum-containing carbonyl ylide 3 a, which then undergoes a [3+2] cycloaddition reaction with n-butyl vinyl ether to give the nonstabilized carbene complex 4 a. Finally, a 1,2-hydrogen-atom shift to the carbene carbon atom gives the product 2 a with regeneration of the catalyst (Scheme 1).

A novel methodology for in situ generation of Fischertype metal−carbene complexes was developed. Photoirradiation to a mixture of an acylsilane and a cationic copper complex cleanly gave a Fischer-type copper−siloxycarbene complex, which was detected by spectroscopic methods. This carbene complex reacted with siloxydienes in a [4 + 1] cycloaddition manner to give cyclopentene derivatives. It is noteworthy that this reaction proceeds with a catalytic amount of copper through in situ generation of a Fischer-type copper− siloxycarbene complex intermediate.

Total synthesis of (±)-Englerin A has been achieved starting from γ,δ-ynone 5 in 14 steps. The key feature of this synthesis is the highly efficient and stereoselective preparation of 8-oxabicyclo[3.2.1]octane derivative 6, a core skeleton of Englerin A, based on an inverse electron-demand [3+2] cycloaddition reaction of the platinum-containing carbonyl ylide, which was developed in our laboratory.

Intermolecular carbon-carbon bond-forming reaction between readily available acylsilanes and aldehydes was achieved under photoirradiation conditions with assistance of a catalytic amount of Lewis acid. Nucleophilic addition of photochemically generated siloxycarbenes to aldehydes followed by 1,4-silyl migration afforded synthetically useful α-siloxyketones. Electrophilic activation of aldehydes by Lewis acid is highly important to realize this reaction efficiently, otherwise the yield of the desired coupling products were significantly decreased. Noteworthy is that a formal cross benzoin-type reaction using acylsilanes was achieved under Lewis acidic conditions. This is the first example of Lewis acid-catalyzed reaction of photochemically generated siloxycarbenes with electrophiles.

Acylsilanes have been knownt ou ndergo isomerizationt os iloxycarbenesu nder photoirradiation and the thus generated carbenes can be utilized for various synthetic reactions. But this carbene formation is not necessarily efficient with some alkanoylsilanes because Norrishtype fragmentations compete, which limit the synthetic utility of alkanoylsilanes as carbene precursors. In this study,g eneration of siloxycarbenes from alkanoylsilanes by visible-light-inducedenergyt ransfer was examined by using an Ir complex, [Ir{dF(CF 3 )ppy} 2 (dtbpy)]PF 6 ,a nd was successfully applied to the CÀCc oupling reactions with boronic esters or aldehydes. This methodology efficiently suppressed undesired Norrish-type reactions and broadened synthetic utility of alkanoylsilanes.Photocatalytic organic transformations have attracted significant attention in currents ynthetic chemistry.P hotoredox process involving ar adicali ntermediate is gaining importance in this field. [1] Another interesting aspect of photocatalytic reactions is triplet energy transfer processes which enableg eneration of triplet-excited states of organic substrates through energy transfer from photoexcited catalysts. [1b,c,e, 2] From as ynthetic point of view,t riplete nergy transfer is considered to have some benefits [3] as compared to direct photoexcitation of organic substrates:i )highly energetic singlet-excited states of substrates, whichs ometimes induce undesired reactions, can be avoided, and ii)ite nables excitationo fs ubstrates with longer wavelength light than the inherent absorption band of substrates.It has been known that triplet-excited states of acylsilanes 1 undergo a1 ,2-silyl migration to generatet ransienta nd nucleo-philic siloxycarbene intermediates 2 (Scheme 1a). [4] Since Brook and co-workers originally discoveredt his photoisomerization through reaction of siloxycarbene intermediates 2 with polar compounds such as alcohols over fifty years ago, [5] various synthetic reactions utilizingp hotochemically generated siloxycarbenes 2 have been developed by several groups [6] including us. [7] However, in contrast to the reactions of aroylsilanes (1, R 1 = aryl), the use of alkanoylsilanes (1,R 1 = alkyl) has been quite limited. [8] We have already reported intermolecularc oupling reactions of photochemically generated siloxycarbenes 2 with boronic esterso raldehydes under neutral or Lewis acidic conditions (Scheme 1b). [7] Duringt he course of these studies, we noticed that serious undesired photochemical processes competed with siloxycarbene formation in the reactions with somea lkanoylsilanes.F or example, UV irradiation( 365 nm) of am ixture of 4-phenylbutanoylsilane 1a [9] and boronic ester 5 gave styrene along with the desired coupling product 3a in low yield (Scheme 2). The formation of styrene clearly indicated that Norrish type II fragmentation [10,11] competed with the desired photochemical isomerization to siloxycarbene 2 from excited stateso falkanoylsilane 1a.Scheme1.a) Photochemical generation of siloxycarbenes. b) Photoi...

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A reliable strategy for the construction of trans-fused bicyclo[n.3.0] skeletons was explored by utilizing photoinduced cyclization of acylsilanes bearing a boronate. The substrates having an acylsilane and a boronate in a 1,2-trans relationship were prepared via hydroboration of cycloalkene derivatives. The desired cyclization was efficiently promoted by photoirradiation to give the trans-fused bicyclo[n.3.0] derivatives as a single diastereomer. It is noteworthy that this methodology enables the efficient construction of a highly strained trans-5-5 ring system.