In this work, coupling of allenylphosphine oxides with aromatic tosylhydrazones were successfully developed under palladium catalysis, delivering (Z)-[3]dendralene derivatives in medium to good yields with high stereoselectivity. Different from the simple coupling of alkyl substituted tosylhydrazones and alkenes, aromatic tosylhydrazones were used as substrate in this work, with which two C=C bonds were constructed simultaneously involving a 1,3-palladium migration process. Under the optimized reaction conditions using Pd(PPh₃)₂Cl₂ as catalyst, anhydrous methanol as solvent and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as base, the reaction showed wide substrate scope and high stereoselectivity. 31 examples of (Z)-[3]dendralenes were obtained with up to 79% isolated yield and up to >20:1 Z/E selectivity. The protocal provideds a simple and efficient method for the synthesis and application of (Z)-[3]dendralene derivatives. Further transformation of compound 3al was achieved to consturct the chromene skeleton which is widely occured in biologically active molecules and natural products. The general procedure is as following: to a Schlenk tube was added allenylphosphine oxide 1 (0.3 mmol), aromatic tosylhydrazone 2 (0.6 mmol), Pd(PPh₃)₂Cl₂ (5 mol%, 11 mg), and ^tBuCOONa•H₂O (0.6 mmol, 85 mg). The reaction flask was sealed, evacuated and back-filled with nitrogen for three times. Afterwards, anhydrous methanol solvent (3 mL) was added through a syringe and DBU (0.9 mmol) was injected slowly. The reaction was carried out at 85 ℃ for 2~3 h. Upon completion of the reaction, the solvent was evaporated under reduced pressure and the residue was purified by column chromatography on silica gel using petroleum ether/ethylacetate (V/V=1:1~2:1) to afford the target product 3. The general procedure for transformation of 3al is as following: to a 10 mL round-bottom flask containing 3al (0.3 mmol), 3 mL of anhydrous dichloromethane was added under argon protection. After cooled to –78 ℃, boron tribromide was added dropwise and the mixture was sitrred for 20 min before moved to room temperature and stirred overnight. The reaction solution was quenched with ice water, extracted with dichloromethane and saturated brine, evaporated under reduced pressure, after which the crude product was separated by column chromatography using petroleum ether/ethylacetate (V/V=2:1) to afford the target product.

Key words: (Z)-[3]dendralenes, aromatic tosylhydrazones, allenylphosphine oxides, palladium catalysis, stereoselectivity