Exclusive weak B decays involving $\tau$ lepton in the relativistic quark model

Semileptonic and leptonic B decays are analyzed in the framework of the relativistic quark model. Special attention is payed to the decays involving $\tau$ lepton. It is found that the calculated particular decay branching fractions are consistent with available experimental data within error bars. However, the predicted and recently measured ratios $R(D^{(\ast )})$ of the $B\to D^{()}\tau\nu_\tau$and$B\to D^{()}l\nu_l$ branching fractions differ by 1.75$\sigma$ for $R(D)$ and by 2.4$\sigma$ for $R(D^{\ast })$.