Create a approved PO from a approved requisition

We have a requirement to create a approved PO once requisition is approved.CREATEPO workflow is present but we don't want to setup ASL ,sourcing rules and quotation to get a standard approved PO.

1st Way

I am using PO_INTERFACE_S.create_documents api to create PO from requisiton.PO is getting generated but the status is 'INCOMPLETE'. So I used two more APIs to reserve and approve PO through backend(PO_documents_funds_pvt.do_reserve and po_documents.do_approve).Here PO is getting approved but it is not getting reserved.Sometimes the reserve API is working sometimes not.Already set up the mo_context_policy.Can anybody suggest what more is to be done to reserve PO from backend in order to call  this