Chromene chromium carbene complexes in the syntheses of naphthopyran and naphthopyrandione units present in photochromic materials and biologically active natural products

The carbene complex 5-(2,2-dimethyl-2H-chromene)methoxylmethylene chromium pentacarbonyl will undergo a benzannulation reaction with phenylacetylene, 1-pentyne, 3-hexyne, and trimethylsilylacetylene to give 7-hydroxy-10-methoxy-3H-naphtho2.1-b]pyrans as the primary product. These compounds are difficult to obtain pure due to their sensitivity to air. If the benzannulation reaction is performed in conjunction with protection of the phenol function at C-7, then good to excellent yields of 7-alkoxy-10-methoxy-3H-naphtho2.1-b]pyrans are afforded. If the 7-hydroxy products are captured by triflic anhydride, then the resulting aryl triflate can be used to access 3H-naphtho2.1-b]pyrans bearing C-7 carbon substituents. The 7-hydroxy products can be oxidized to 3H-naphtho2,1-b]pyran-7,10-diones which are stable. The chromenyl carbene complex reacts with 1,6-bis(triisopropylsilyl)-1,3,5-hexatriyne to give a 2,3-dihydro-2,2-dimethylbenzode]chromene, a product type that has not been seen before in the reaction of Fischer carbene complexes with alkynes. A mechanism is proposed for this process that involves alpha,beta-hydride elimination from a chromacyclobutane intermediate. Chromenyl tungsten complexes react with alkynes to give products that result from cyclization without CO insertion.