Wangqing Kong

An efficient synthesis of carbazoles from PtCl2-catalyzed cyclization of 1-(indol-2-yl)-2,3-allenols

The PtCl(2)-catalyzed reaction of 1-(indol-2-yl)-2,3-allenols occurred smoothly to form carbazoles by connecting the 3-carbon atom of indole with the 4-carbon atom of the allenol moiety, referring to the carbon atom connected to the hydroxyl group.

PtCl4-Catalyzed Cyclization Reaction of β-Allenols in the Presence of Indoles

The highly regioselective PtCl4-catalyzed reaction of indoles with beta-allenols in THF at room temperature afforded indole derivatives containing a six-membered ether ring at the 3-position in moderate isolated yields. On the basis of a D-labeling experiment, a mechanistic rationale was proposed.

Regiocontrolled 1,2-migration in cyclization of 1-(indol-2-yl)-3-alkyn-1-ols: (Ph3P)Au+vs. PtCl4

An AuCl(PPh3)/AgBF4- and PtCl4-catalyzed reaction of 1-(indol-2-yl)-3-alkyn-1-ols occurred smoothly in toluene to form a series of differently polysubstituted carbazole derivatives efficiently. The regioselectivity of the 1,2-migration may be tuned by using different metal catalysts: carbazoles 3 could be obtained exclusively in the presence of AuCl(PPh3)/AgBF4via a Wagner–Meerwein type 1,2-alkyl shift, whereas in some cases the use of PtCl4 afforded differently substituted carbazoles 4 involving a platinum–carbene intermediate.

An Efficient CuCN-Catalyzed Synthesis of Optically Active 2,3-Allenols from Optically Active 1-Substituted 4-Chloro-2-butyn-1-ols

The sequential treatment of optically active terminal propargylic alcohols with n-BuLi/(HCHO)(n) and regioselective chlorination afforded the corresponding optically active 4-chloro-2-butyn-1-ols. With R(1) being a methyl or an ethyl group, an alternative for the synthesis of the corresponding optically active propargylic alcohols is the Novozym 435-catalyzed kinetic resolution of these racemic 4-chloro-2-butyn-1-ols. The subsequent reaction of these optically active 4-chloro-2-butyn-1-ols with the corresponding Grignard reagents under the catalysis of 5 mol % of CuCN afforded the optically active secondary 2,3-allenols in good yields with up to >99% ee.

Studies on electrophilic reaction of tertiary 2,3-allenols with NBS in H2O or aqueous MeCN: an efficient selective synthesis of 2-bromoallylic ketones, 1,2-allenyl ketones, or 3-bromo-2,5-dihydrofurans.

Previously, we observed that the electrophilic reaction of 2,3-allenols with X(+) (X = Br, I) affords 3-halo-2-alkenals or 2-halo-2-alkenyl ketones in aqueous MeCN (MeCN/H(2)O = 15:1). However, the reaction of tertiary 2,3-allenols with NBS under these reaction conditions affords a mixture of rearrangement products (aldehydes or ketones) together with 1-bromovinyl epoxides in a low selectivity. Due to the synthetic potential of 2-haloallylic ketones, we decided to explore this reaction further. After screening, we observed that the electrophilic reaction of terminal tertiary 2,3-allenols with NBS in water affords 2-bromoallylic ketones highly selectively in up to 97% yields via a sequential electrophilic interaction of Br(+) with the allene moiety to form a possible three-membered bromonium intermediate with the more substituted C=C bond, which would be followed by 1,2-aryl or 1,2-alkyl shift to open the strained three-membered ring and proton elimination to form the carbonyl functionality. When both R(1) and R(2) (the substituents on the carbon atom connected to the hydroxyl groups) are alkyl groups, one of these two groups is migrated; with 1,2-propadienyl cycloalkanols, a ring expansion reaction was observed; with R(1) being an aryl group, R(1) would be transferred exclusively to form the 2-bromoallylic ketones. Interestingly, these 2-bromo-1-aryl-2-propenyl ketones may easily be converted into 1,2-allenyl ketones after column chromatography on silica gel pre-eluented with 10 drops of Et(3)N; when there is at least an alkyl substituent on the 4-position of the tertiary 2,3-allenols, their electrophilic reaction with NBS in CH(3)CN/H(2)O = 15/1 or H(2)O, under the same reaction conditions as above, affords 3-bromo-2,5-dihydrofurans in 61-84% yields, indicating that the electronic effect and the steric effect of the two C=C bonds determine the reaction pathway, i.e., the Br(+) interacts with the C=C bond at the 3-position.

Indium and zinc-mediated Barbier-type addition reaction of 2,3-allenals with allyl bromide: an efficient synthesis of 1,5,6-alkatrien-4-ols.

A zinc or indium-mediated Barbier-type addition reaction of 2,3-allenals with allyl bromide in a mixed medium of aqueous NH(4)Cl and THF (5:2) was developed to provide an efficient route to 1,5,6-alkatrien-4-ols, which is synthetically very useful. No 1,4-addition reaction was observed. Depending on the substrates, both indium and activated zinc afforded the 1,2-addition products in moderate to excellent yields: for terminal allenals, activated zinc was better while in other cases the yields with indium were relatively higher.