First observations of the Cabibbo-suppressed decays ¯¯¯ B 0 → D + K − π + π − and B − → D 0 K − π + π − are reported using 35     pb − 1 of data collected with the LHCb detector. Their branching fractions are measured with respect to the corresponding Cabibbo-favored decays, from which we obtain B ( ¯¯¯ B 0 → D + K − π + π − ) / B ( ¯¯¯ B 0 → D + π − π + π − ) = ( 5.9 ± 1.1 ± 0.5 ) × 10 − 2 and B ( B − → D 0 K − π + π − ) / B ( B − → D 0 π − π + π − ) = ( 9.4 ± 1.3 ± 0.9 ) × 10 − 2 , where the uncertainties are statistical and systematic, respectively. The B − → D 0 K − π + π − decay is particularly interesting, as it can be used in a similar way to B − → D 0 K − to measure the Cabibbo-Kobayashi-Maskawa phase γ .