Mscrm 2011 C# code to close the opportunity as Lost :

 public void CloseOpportunitiesAsLost(IOrganizationService service, Guid opportunityId)

        {

            Entity entity = new Entity("opportunityclose");

            entity["opportunityid"] = new EntityReference("opportunity", opportunityId);

            entity["actualend"] = DateTime.Now;

            LoseOpportunityRequest woReq = new LoseOpportunityRequest();            

            woReq.OpportunityClose = entity;

            woReq.Status = new OptionSetValue(-1);

            service.Execute(woReq);

        }