Hello everyone.
The following post as been closed but, since we found the solution, we would like to share it because it might help someone in the future.
https://developer.mypurecloud.com/forum/t/adfs-oauth-saml2-bearer-authentication/1777
Basically, when developing a custom front-end the whole ADFS negotiation needs to be done between ur custom application and ADFS. PureCloud joins the equation only to get a Oauth token from the previously obtained SAML response. The relying trust party for the custom front-end can be configured as the one for PureCloud, except for the ACS URL's that must belong to the custom application. Also important, the SAML Request must specify the ACS and it must match the one on the ADFS side.
Finally, a very helpful tool to achieve this integration was a Firefox plugin call SAML Tools. It allows u to inspect the SAML negotiation and debug it.
Best regards,
Daniel Grosso