Gold‐Catalyzed Tandem Cyclization of Dienol Silyl Ethers for the Preparation of Bicyclo[4.3.0]nonane Derivatives

Abstract

We previously reported the geminal carbo-functionalization reaction of alkynes on treatment of 3-siloxy-1,3-dien-7-ynes 1 with [W(CO) 6 ] or [ReCl (CO) 5 ] under photoirradiation to give bicyclo[3.3.0]octane derivatives 2 in good yield (Scheme 1). [1] The reaction was thought to proceed through nucleophilic attack of the silyl enol ether moiety to the electrophilically activated alkyne followed by attack of the generated alkenyl metallic species A to the a,b-unsaturated silyloxonium moiety at the position b to the metal to give bicyclic carbene complex intermediate B, which gave the product through 1,2-hydrogen shift with regeneration of the catalyst. [2] We then thought of the possibility of carrying out a similar geminal carbofunctionalization reaction by employing one-carbon-atom elongated substrates (3-siloxy-1,3-dien-8-ynes 3) with the expectation that 5-exo attack of the silyl enol ether moiety followed by ring closure (geminal carbo-functionalization) would give bicyclo[3.3.0]octane derivative D having a carbene complex moiety as a substituent on the bridgehead carbon atom. As there is no a-hydrogen atom at the carbene complex moiety, 1,2-hydrogen shift is not possible and the behavior of this intermediate is intriguing (Scheme 1). We now report that 1,2-alkyl migration [3] occurred at the generated carbene moiety to give ring-expanded products by employing 3siloxy-1,3-dien-8-yne derivatives as substrates, to stereoselectively obtain synthetically useful bicyclo[4.3.0]nonane derivatives, whose configuration was different from that of the Diels-Alder adduct.We first examined the reaction of 3-siloxy-1,3-dien-8-yne 3 a with the geometry of the silyl enol ether moiety as (Z) [4] by using various electrophilic transition-metal catalysts. Although [W(CO) 5 (thf)] or [ReCl(CO) 5 ] gave the products in reasonable yield (45 % and 78 %, respectively), [5] the cationic gold catalyst gave the best result. Thus, treatment of 3 a with 10 mol % of [AuClPPh 3 ]/AgSbF 6 [6] in the presence of molecular sieves (4 ) gave bicyclo[4.3.0]nonane derivative 4 a, a formal [4+2] cycloadduct, as a single stereoisomer in 92 % yield (Scheme 2). [7,8] Interestingly, the configuration of the product was different from that of the thermal Diels-Alder adduct 5 obtained stereoselectively by heating a solution of (Z)-3 a in toluene for 1.5 h. [9] Thus, two diastereoisomers [10] of the same bicyclo[4.3.0]nonane derivative were obtained selectively by choosing Au I -catalyzed or thermal conditions. Next, we examined the reaction of 3-siloxy-1,3-dien-8-yne derivative 3 a with the geometry of the silyl enol ether moiety as (E) [4] under thermal and Au I -catalyzed conditions (Scheme 2). In this case, (E)-3 a did not undergo thermal Scheme 1. Tandem cyclization of dienol silyl ethers.Scheme 2. Au I -catalyzed and thermal cyclization of dienynes 3 a. E = CO 2 Me; TIPS = triisopropylsilyl; M.S. = molecular sieves; DCE = dichloroethane.

Tungsten(0)‐ and Rhenium(I)‐Catalyzed Tandem Cyclization of Acetylenic Dienol Silyl Ethers Based on Geminal Carbo‐Functionalization of Alkynes

“…Surprisingly, treatment of 11 a with [ReCl(CO) 5 ] (10 mol %) gave a 78 % yield of the tricyclic compounds 13 a and 14 a (Table 10), produced by insertion of the carbene complex moiety into the neighboring C À H bonds in the carbene complex intermediate generated by 5-exo cyclization. [25] No other cyclized products were obtained. This high insertion ability of the carbene complex intermediate is a remarkable characteristic of the rhenium(I) catalyst.…”

“…We had already reported that on use of the cationic gold(I) catalyst, the dienol silyl ethers 11 a diastereoselectively gave the bicycloA C H T U N G T R E N N U N G [4.3.0]nonane derivatives 12 a (Scheme 10), with different stereochemistry from the thermal Diels-Alder products. [25] In these reactions, geminal carbo-functionalization through initial 5-exo cyclization gave a bicyclic carbene complex such as I, which underwent 1,2-alkyl migration of the benzylic carbon to give the product with regeneration of the catalyst. [26] During these studies, we thought of the possibility-based on previous experimental results [27] -of geminal carbo-functionalization through initial 6-endo cyclization in the presence of tungsten catalyst.…”

A 11‐Steps Total Synthesis of Magellanine through a Gold(I)‐Catalyzed Dehydro Diels–Alder Reaction

“…( 3)]. [9] These ubiquitous structural scaffolds are embedded in ab road variety of naturally occurring complex alkaloids and terpenes such as magellanine (7), dankasterone (8)a nd conidiogenol (9), to name af ew.H erein, we report an efficient and stereoselective Au I -catalyzed DDAreaction that enables the preparation of angular carbocycles and its application to ashort synthesis of (AE)-magellanine (7).…”