you can via SPG, your options are:
1> 180 AMEX MR --> 90K SPG --> 270K Marriott --> 120K AP or most FF (or 132K UA) + 7-night CAT 1-5 Cert
2> 40K AMEX MR --> 20K SPG --> 25K AP or some FF (noted for UA, only get 12.5K with this method)
True, but those are terrible conversion rates; especially since the AMEX gold and platinum convert to AP and BA 1:1