The present invention discloses an improved process for preparation of substantially pure pentamethylene bis(1-(3,4-dimethoxybenzyl)-3,4-dihydro-6,7- dimethoxy-1H-isoquinoline-2-propionate), a compound of formula I and its acid addition salts in good yields. Pentamethylene bis(1-(3,4-dimethoxybenzyl)-3,4- dihydro-6,7-dimethoxy-1H-isoquinoline-2-propionate) or its acid addition salts is the key intermediate for the synthesis of a non-depolarizing neuromuscular-blocking drugs. The process can also be used for the synthesis of various tetrahydroisoquinoline derivatives.