Inverting negative-to-positive step-down switching converter
Negative input power can be converted to positive output with following ICs:

This schematics was found on TI forum (notice the source link on the scheme).

Following method uses less components and found in the datasheet of each chip accordingly.

LT1070, LT1071, LT1170, LT1171, LT1172:


And minimal possible components with small coil and handy package S8E (For 3.3V output set R4 approx. 32k):