  We proceed with the investigation of a method of quantization of the
observable sector of closed bosonic strings. For the presentation of the
quantum algebra of observables the construction cycle concerning elements of
order \hbar^6 has been carried out. We have computed the quantum corrections to
the only generating relation of order \hbar^6. This relation is of spin-parity
J^P=0^+. We found that the quantum corrections to this relation break the
semidirect splitting of the classical algebra into an abelian, infinitely
generated subalgebra a and a non-abelian, finitely generated subalgebra U. We
have established that there are no ("truly independent") generating relations
of order \hbar^7.
